We generalize the necessary and sufficient conditions for finite systems to exhibit van der Waals-type loops (or "S-bends") in thermodynamic functions. In the infinite limit, such features are forbidden by Van Hove’s theorem, and phase separation occurs instead. However, in small systems the energetic cost of the interface associated with phase separation is too great, and there is no contribution to the partition function from regions of phase space corresponding to two phases coexisting in contact. We derive a simple model that can correctly describe both limits, and investigate how the onset of phase separation will affect observable thermodynamic properties.