Inflationary models that depend on a temperature-dependent phase change require a homogeneous thermal state as an initial condition. In this paper we estimate the average total number of interactions per particle obtained with various models and thereby determine self-consistently whether a thermal state is likely. The requirement that a thermal state should occur is used to provide bounds on the parameters in such models. The formalism we develop can be used to study questions concerning the validity of the initial thermal state in a quantitative way. In particular we verify that the new inflation'' scenario is not viable.