1-Alkoxymethylvinylphosphonates3a-ehave been synthesized by piperidine catalyzed condensation of diethylphosphonoacetic acid1with paraformaldehyde in the presence of various primary and secondary alcohols. Similar reaction of the acid1with paraformaldehyde and sorbic alcohol3fafforded a mixture of three bicyclic compounds7, 9and11in a ratio 2:1:1, respectively. The latter results strongly evidence the intermediacy of phosphonoacrylic acid4in the condensation. Under analogous conditions the olefin14was obtained from the acid (E,Z)-13.