P,P-Dimethyl-C-alkoxy-methylenamino-phosphines 7 react with electrophilic olefines 8 such as acrylic, crotonic, cinnamic acid derivatives and α,β-unsaturated ketones in a 1,3-dipolar cycloaddition and immediate elimination of alcohol to give the title compounds 10. 1H- and 31P-NMR data prove their structure and show the electronic interaction of the P=N and C=C ring fragments. Contrasting to pyrrole, where the cyclic π-system stabilises the enamine form, the prototropic equilibrium of its phospha analogues 10 is completely on the side of the imine form.