diff --git a/conservancy_beancount/data.py b/conservancy_beancount/data.py
index b93a2c7e854b4beba00ff94d7062980142f599bb..7c016e00617578516f39be45c4ad4a5e0475cccc 100644
--- a/conservancy_beancount/data.py
+++ b/conservancy_beancount/data.py
@@ -25,6 +25,7 @@ import operator
 
 from beancount.core import account as bc_account
 from beancount.core import amount as bc_amount
+from beancount.core import convert as bc_convert
 
 from typing import (
     cast,
@@ -257,18 +258,28 @@ class Posting(BasePosting):
 
 def balance_of(txn: Transaction,
                *preds: Callable[[Account], Optional[bool]],
-) -> decimal.Decimal:
+) -> Amount:
     """Return the balance of specified postings in a transaction.
 
     Given a transaction and a series of account predicates, balance_of
     returns the balance of the amounts of all postings with accounts that
     match any of the predicates.
+
+    balance_of uses the "weight" of each posting, so the return value will
+    use the currency of the postings' cost when available.
     """
-    return sum(
-        (post.units.number for post in iter_postings(txn)
-         if any(pred(post.account) for pred in preds)),
-        decimal.Decimal(0),
-    )
+    match_posts = [post for post in iter_postings(txn)
+                   if any(pred(post.account) for pred in preds)]
+    number = decimal.Decimal(0)
+    if not match_posts:
+        currency = ''
+    else:
+        weights: Sequence[Amount] = [
+            bc_convert.get_weight(post) for post in match_posts  # type:ignore[no-untyped-call]
+        ]
+        number = sum((wt.number for wt in weights), number)
+        currency = weights[0].currency
+    return Amount._make((number, currency))
 
 def iter_postings(txn: Transaction) -> Iterator[Posting]:
     """Yield an enhanced Posting object for every posting in the transaction"""