The oxidative dehydrogenation (ODH) of ethane to ethylene over stoichiometric iron phosphate phases, e.g. FePO4, Fe2P2O7, alpha-Fe-3(P2O7)(2) and beta-Fe-3(P2O7)(2), and nonstoichiometric (mixed) iron phosphate phases with P:Fe ratios of 1.2:1 and 2:1 was examined at temperatures ranging from 400 to 675degreesC. The phosphates tend to be unstable under reaction conditions. and generally convert to the quartz-like FePO4 and other phases, depending on stoichiometry. Under certain conditions, the nonstoichiometric phases exhibit an unusual activation behavior in that both selectivity and conversion increase over time. Excellent results were achieved over the active forms of the nonstoichiometric catalysts; ethylene yields approach 50% over a material with a P:Fe ratio of 1.2:1. We were unable to correlate a single crystalline or amorphous phase with catalytic performance. Rather, the evidence suggests that the activity and selectivity results from the migration of phosphorus out of the bulk iron phosphate material to the catalyst and reactor surfaces. Published by Elsevier Science B.V.