The effect of nitrogen addition in the 0-1.0 at.% range on the mean lamellar thickness of a fully lamellar TiAl alloy (Ti-48.5Al-1.5Mo) has been studied. Mean lamellar thickness first decreases with 0.3 at.% N addition but then increases with further increase in N level in the alloy. The increase in lamellar spacing for N levels between 0.3 and 1.0 at.% is attributed to the formation of Ti2AlN nitrides and the consequent change in Al level in the matrix. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.