Attempts at preparing cyclopropenylphosphonium salts by P-quaternization of α-aminophosphines with bisdiisopropylaminocyclopropenium tetrafluoroborate led to azetidin-2-ylidene methylphosphoniums as unexpected isomers, in 57-85% yields. The basic structure of the products is discussed on the basis of X-ray crystallography analysis. The unprecedented 3 → 4 ring expansion process is argued to be driven by an ambivalent aromatic vs antiaromatic character (or loss of aromaticity) of the primary phosphonium product. On the basis of DFT calculations, a mechanism involving a concerted 1,4-hydride shift/electrocyclization process as the rate-determining step is proposed and discussed vs possible alternatives.