Some transitions between ordered and disordered magnetic states that take place via transitions of first order are tabulated. Among these transitions are some of the order‐disorder type. The theoretical basis for such a transition to occur is indicated using a simple physical model that assumes: (1) a strain dependent exchange interaction and (2) a compressible lattice. The necessary condition for first‐order behavior is determined, and the theory is compared to the behavior of a real material, MnAs. It is found to be in substantial agreement with the behavior of that compound. The system represented by the simple model employed here possesses ferro‐, antiferro‐, and paramagnetic states. The equilibrium boundaries between these states are determined in the pressure‐temperature plane as an illustration of the possible transitions that may be observed with this model.