1. Independent of the solvent and the proton acceptor, a mixture of the cis-trans isomers of ethyl α-ionylideneacetate is formed when (C2H5)2P(O)CH2OOC2H5 is reacted with α-ionone. 2. Independent of the proton acceptor (NaNH2 or NaH), the trans-isomer of ethyl β-ionylideneacetate is formed predominantly when (C2H5)2P(O)CH2COOC2K5 is reacted with β-ionone in tetrahydrofuran medium, while a mixture of the cis-trans isomers is obtained in dimethylformamide medium (proton acceptor -C2H5ONa, NaH or NaNH2).