  The phase transition of a fluid adsorbed in a heterogeneous system is studied
with two simple lattice gas models within the framework of a mean-field theory.
Despite the different origin of the heterogeneity (spatial variation of binding
energy or fluid coordination numbers), the fluid can undergo two phase
transitions if the hosting system is sufficiently heterogeneous. It is clearly
shown that such a polymorphism in the confined fluid results from the
successive condensations in distinct spatial regions of the host. We have found
the precise conditions at which one two phase transitions occur. The insight
gained from the present study allows one to understand better some recent
puzzling simulation results.
