diff --git a/doc/formats.md b/doc/formats.md
index 32a0482..ab01d35 100644
--- a/doc/formats.md
+++ b/doc/formats.md
@@ -124,3 +124,21 @@ are used to set initial values for runtime parameters:
 * apikeys-file
 * jendasap-checked
 * bank-dir
+
+API Keys File
+-------------
+
+Each line represents one Fio account.
+
+There must be no leading whitespace on any line. No comments or empty
+lines are allowed.
+
+Tokens on a line are separated by spaces.
+
+Currently each line must have at least three tokens and only these
+three tokens are used:
+
+* account number
+* Fio API key
+* starting year since when this account is synchronized
+