Local thermal equilibrium refers to the state in which a single temperature can be used to describe a heat transfer process in a multiphase system. When this condition occurs, a one-equation model can be used and the analysis of the heat transfer process is greatly simplified. In this paper we first develop the constraints that must be satisfied in order that the principle of local thermal equilibrium be valid, and we then compare these constraints with numerical experiments for transient heat conduction in two-phase systems. Reasonable agreement between the estimates and the numerical experiments is obtained.