The model describing the behavior of precipitates of variable composition at heat treatment is suggested. The specific feature of this model is that it enables to take into account the effect of diffusion inside the particles on the kinetics of the process. The application of this model is demonstrated by the example of vanadium carbonitrides V(C,N) evolution in austenite at heat treatment of vanadium-containing steels. The necessity to take into account the diffusion modification of carbonitride composition is considered. Examples of particles composition evolution at heat treatment are demonstrated.