The dynamic behavior of a simple heterogeneous catalytic system is examined, and necessary and sufficient conditions for a lumped model to predict sustained oscillatory behavior are found. These conditions are equivalent to the system's exhibiting a possible thermodynamic phase transition. The compatibility of the asymptotic and thermodynamic stability conditions for the uniform steady states at the limit of adsorption-desorption equilibrium is discussed. It is shown that the asymmetric steady states resulting from phase separation are conceptually different from those caused by reaction anomalies. For a Bragg-Williams approximation to adsorbate interactions the functional dependence of the rate on the nonuniform surface coverage is established by use of the mean-field theory of inhomogeneous fluids.