We introduce varying approximations to the thermodynamic properties of a convecting system similar to Earth's mantle. A realistic geometry, rheology and faults are included to insure that the variations we see are robust in a dynamic, self-buffering environment. We find two levels of response in the system. First, some approximations lead to very different patterns of flow in the simulations. Second, other sets of approximations lead to similar patterns of flow, but the time history of the flow varies significantly with the formulation used. Last, we note that only minor changes to an existing code would be needed to integrate a complex thermodynamic model. ¿ 1998 American Geophysical Union