A phenomenological model for the unbinding transition of multicomponent fluid membranes is proposed, where the unbinding transition is described using a theory analogous to Flory‐Huggins theory for polymers. The coupling between the lateral phase separation of inclusion molecules and the membrane‐substrate potential provides a rich phase behavior. Our model describes the first‐order nature of the unbinding transition in multicomponent membranes as was observed in a recent experiment. In particular, we predict different scenarios of phase coexistence between bound and unbound membrane states. © 2004 American Institute of Physics