Iminophosphoranes of the type X3P=NR (X=Cl, pyrrolyl; R=alkyl, aryl) catalytically metathesize C=N bonds of carbodiimides via an addition/elimination mechanism that, despite the lack of d orbital participation in P-N bonding, conserves the key features of metal-catalyzed olefin metathesis. Diazaphosphetidine intermediates, produced by the formal [2+2] addition of carbodiimides to the P=N bond, have been isolated and characterized. All phosphorus-containing species in the complex catalytic reaction mixtures have been identified and their origins explained. The kinetics of addition of diisopropylcarbodiimide to Cl3P=NPl and subsequent elimination were studied, and rate constants were determined: k(add)=1.7x10(-3) (+/-0.1x10(-3)) M s(-1) and k(elim)=4.0x10(-4) (+/-0.3x10(-4)) s(-1). The rate of these reactions corresponds well with the observed catalytic TOF of 1.44 TO/P/h.