diff --git a/minter/src/state/mod.rs b/minter/src/state/mod.rs
index 36e97d10..c265d456 100644
--- a/minter/src/state/mod.rs
+++ b/minter/src/state/mod.rs
@@ -3,7 +3,9 @@ use crate::{
     ledger::client::LedgerClient,
     numeric::{LedgerBurnIndex, LedgerMintIndex},
     state::event::{DepositId, TransactionPurpose, VersionedMessage, WithdrawalRequest},
-    utils::insertion_ordered_map::InsertionOrderedMap,
+    utils::{
+        insertion_ordered_map::InsertionOrderedMap, insertion_ordered_set::InsertionOrderedSet,
+    },
 };
 use candid::Principal;
 use cksol_types::{DepositStatus, TxFinalizedStatus, WithdrawalStatus};
@@ -89,7 +91,7 @@ pub struct State {
     minimum_deposit_amount: Lamport,
     process_deposit_required_cycles: u128,
     deposit_consolidation_fee: u128,
-    monitored_accounts: BTreeSet<Account>,
+    monitored_accounts: InsertionOrderedSet<Account>,
     pending_process_deposit_request_guards: BTreeSet<Account>,
     pending_withdrawal_request_guards: BTreeSet<Account>,
     accepted_deposits: InsertionOrderedMap<DepositId, Deposit>,
@@ -244,7 +246,7 @@ impl State {
         self.balance
     }
 
-    pub fn monitored_accounts(&self) -> &BTreeSet<Account> {
+    pub fn monitored_accounts(&self) -> &InsertionOrderedSet<Account> {
         &self.monitored_accounts
     }
 
@@ -776,7 +778,7 @@ impl TryFrom<InitArgs> for State {
             minimum_deposit_amount,
             process_deposit_required_cycles: process_deposit_required_cycles as u128,
             deposit_consolidation_fee: deposit_consolidation_fee as u128,
-            monitored_accounts: BTreeSet::new(),
+            monitored_accounts: InsertionOrderedSet::new(),
             pending_process_deposit_request_guards: BTreeSet::new(),
             pending_withdrawal_request_guards: BTreeSet::new(),
             accepted_deposits: InsertionOrderedMap::new(),
diff --git a/minter/src/state/tests.rs b/minter/src/state/tests.rs
index a6afa737..e3ef55e3 100644
--- a/minter/src/state/tests.rs
+++ b/minter/src/state/tests.rs
@@ -17,7 +17,9 @@ use crate::{
         runtime::TestCanisterRuntime,
         signature, sol_rpc_canister_id, valid_init_args,
     },
-    utils::insertion_ordered_map::InsertionOrderedMap,
+    utils::{
+        insertion_ordered_map::InsertionOrderedMap, insertion_ordered_set::InsertionOrderedSet,
+    },
 };
 use assert_matches::assert_matches;
 use cksol_types_internal::{Ed25519KeyName, InitArgs, SolanaNetwork, UpgradeArgs};
@@ -226,7 +228,7 @@ mod state_from_init_args {
                 minimum_withdrawal_amount: MINIMUM_WITHDRAWAL_AMOUNT,
                 minimum_deposit_amount: MINIMUM_DEPOSIT_AMOUNT,
                 process_deposit_required_cycles: PROCESS_DEPOSIT_REQUIRED_CYCLES,
-                monitored_accounts: BTreeSet::new(),
+                monitored_accounts: InsertionOrderedSet::new(),
                 pending_process_deposit_request_guards: BTreeSet::new(),
                 pending_withdrawal_request_guards: BTreeSet::new(),
                 accepted_deposits: InsertionOrderedMap::new(),
diff --git a/minter/src/utils/insertion_ordered_set.rs b/minter/src/utils/insertion_ordered_set.rs
new file mode 100644
index 00000000..2b775aba
--- /dev/null
+++ b/minter/src/utils/insertion_ordered_set.rs
@@ -0,0 +1,60 @@
+use super::insertion_ordered_map::InsertionOrderedMap;
+
+/// An insertion-ordered set backed by an [`InsertionOrderedMap`].
+///
+/// Provides O(log n) membership tests and O(log n) insertion/removal,
+/// while preserving insertion order during iteration.
+pub struct InsertionOrderedSet<K>(InsertionOrderedMap<K, ()>);
+
+impl<K: Ord + Clone> InsertionOrderedSet<K> {
+    pub fn new() -> Self {
+        Self(InsertionOrderedMap::new())
+    }
+
+    /// Inserts `key`. Returns `true` if the key was not already present.
+    pub fn insert(&mut self, key: K) -> bool {
+        self.0.insert(key, ()).is_none()
+    }
+
+    /// Removes `key`. Returns `true` if the key was present.
+    pub fn remove(&mut self, key: &K) -> bool {
+        self.0.remove(key).is_some()
+    }
+
+    pub fn contains(&self, key: &K) -> bool {
+        self.0.contains_key(key)
+    }
+
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    /// Iterates over keys in insertion order.
+    pub fn iter(&self) -> impl Iterator<Item = &K> {
+        self.0.keys()
+    }
+}
+
+impl<K: Ord + Clone> Default for InsertionOrderedSet<K> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl<K: Ord + Clone> PartialEq for InsertionOrderedSet<K> {
+    fn eq(&self, other: &Self) -> bool {
+        self.0 == other.0
+    }
+}
+
+impl<K: Ord + Clone> Eq for InsertionOrderedSet<K> {}
+
+impl<K: Ord + Clone + std::fmt::Debug> std::fmt::Debug for InsertionOrderedSet<K> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_set().entries(self.iter()).finish()
+    }
+}
diff --git a/minter/src/utils/mod.rs b/minter/src/utils/mod.rs
index cc87b6c5..5ca7437c 100644
--- a/minter/src/utils/mod.rs
+++ b/minter/src/utils/mod.rs
@@ -1 +1,2 @@
 pub mod insertion_ordered_map;
+pub mod insertion_ordered_set;