diff --git a/specification.rst b/specification.rst
index f98a46f49..5bfe8f777 100644
--- a/specification.rst
+++ b/specification.rst
@@ -228,6 +228,8 @@ CheckTx
    After ``Commit``, the mempool will rerun CheckTx on all remaining
    transactions, throwing out any that are no longer valid.
 
+   Keys and values in Tags must be UTF-8 encoded strings (e.g. "account.owner": "Bob", "balance": "100.0", "date": "2018-01-02")
+
 
 DeliverTx
 ^^^^^^^^^
@@ -251,6 +253,7 @@ DeliverTx
 
    - Deliver a transaction to be executed in full by the application. If the transaction is valid,
      returns CodeType.OK.
+   - Keys and values in Tags must be UTF-8 encoded strings (e.g. "account.owner": "Bob", "balance": "100.0", "time": "2018-01-02T12:30:00Z")
 
 EndBlock
 ^^^^^^^^