Co39Ni33Al28 ribbon with a beta phase structure, which exhibited a martensitic transformation to L1(0) phase.. was found to be unstable and decomposed into a beta + gamma phase mixture upon heating to above 973 K. The double-phase structure was found to exhibit no martensitic transformation, despite the presence of the beta phase. This is ascribed to the change of the beta phase composition due to the formation of the gamma phase. Annealing of the decomposed sample at 1473 K, followed by rapid cooling, was effective in restoring the B-2 -> L1(0) martensitic transformation. (c) 2006 Acta Materialia, Inc. Published by Elsevier Ltd. All rights reserved.