The effect of accelerated thermal cycling on a joint between modified 9Cr-1Mo steel (Grade 91) and Alloy 800 welded with Inconel1 82 and 182 filler material is discussed. This is part of a trimetallic transition joint involving Grade 91-Alloy 800-316LN austenitic stainless steel for steam generator application. It has been shown that, during thermal cycling following the typical post-weld tempering treatment at 760°C for 2 h, no carbon diffusion occurs from the ferritic steel towards the weld metal. There is, in fact, a hardness increase at the ferritic steel/weld metal interface which is probably a result of work hardening. Carbon migration sets in only after unusually long post-weld heat treatments for 20 and 50 h at 760°C followed by thermal cycling. Significantly, even under the most severe thermal cycling test conditions imposed, no cracking or oxide notching could be detected, thus demonstrating the superior performance potential of modified 9Cr-1Mo steel as part of a trimetallic configuration. © 2002 Elsevier Science Ltd. All rights reserved.