The iminophosphorane Cl3P=NAr (1a, Ar = 2-fluorophenyl) reacts metathetically with imines at 80 C to produce =NR exchange products. Compound 1a effectively catalyzes imine/imine and imine/carbodiimide cross-metathesis. The observation of =NR exchange products as well as spectroscopic evidence for the existence of diazaphosphetidine type intermediates suggests that a [2 + 2] addition/elimination mechanism is the primary pathway for substrates with N-alkyl substituents and a secondary pathway for N-aryl imines. In contrast to previously studied carbodiimide systems, the resting state of the catalyst is the iminophosphorane and not the diazaphosphetidine. For N-aryl imines, Lewis-acid catalysis appears to be the dominant mechanism, not addition/elimination. For N-alkyl imines, a decomposition pathway, involving HCl elimination from a phosphorus intermediate, is competitive in some cases.