1-Dimethylamino-1-methoxy-1-alkenes (ketene O,N-acetals), 1-dimethylamino-1-ethylthio-1-alkenes (keteneS,N-acetals) and 1,1-bis(dimethylamino)ethylene (ketene N,N-acetal) reacted with diethyl phosphite to give the corresponding (E)-diethyl 1-dimethylamino-1-alkenylphosphonates. Although ketene diethylacetal (ketene O,O-acetal) reacted with diethyl phosphite to give diethyl 1,1-diethoxy-1-ethylphosphonate