A systematic investigation of the dependence of the Verwey phase transition in magnetite on the metal-to-oxygen ratio, by thermomagnetic analysis of the initial permeability, reveals that the transition temperature is a maximum for pure stoichiometric Fe3O4 and is strongly depressed by departures from ideal stoichiometry. Homogeneous single crystalline samples were prepared by subsolidus controlled oxygen fugacity annealing at 1400 °C. Heat capacity measurements by relaxation calorimetry techniques indicate a drastic reduction in the entropy, as well as temperature, of the transition with departures from ideal stoichiometry. For high levels of cation deficiency, a second, instead of a first-order, transition is observed.