Cr-Mo-V rotor steel has been cycled in total axial strain control at room and elevated temperature with and without holds in tension. We have studied by TEM/EDS techniques the changes in carbide morphology and composition produced by these exposures. The carbide morphology has been found to change, even at room temperature, from anisotropic morphology to more equiaxed forms, and the compositions of the carbides have become enriched with Mo and V, at rates which are orders of magnitude greater than would be observed in conventional tempering processes. Naturally the effects of high temperature and tests with holds have been to accelerate these changes. This applies particularly to the redistribution of Mo which can be associated with a possible fatigue-induced decline in ductility. One explanation of this behavior is the elimination of the impurity-scavenging action of the Mo as it is absorbed into the previously existing carbides and as it forms new carbides.