N-phenyliminovinylidenetriphenylphosphorane enter cycloaddition reactions with vinyl isocyanate and vinyl isothiocyanates and giving exclusively [4+2] cycloadditionproducts, i.e. derivatives of 3-triphenylphosphoranopyrid-2-one and or 3-triphenylphosphoranopyridine-2-thione (, and ), in contrast with alkyl or aryl isocyanates or isothiocyanates. The structure of the obtained cycloadducts was determined