Taking the reaction of diethyl 3-ethoxycarbonyl-2-methyl-2-propenylphosphonate with 3-methylbutanal as an example, it has been shown that the composition of the equilibrium mixture of the E- and Z-isomers of the starting phosphonate formed in the presence of bases is independent of the base anion (KOH or K2CO3) and the solvent (benzene or DMSO). In the mixture of 2E,4E- and 2Z,4E-isomers of ethyl 3, 7-dimethyl-2, 4-octadienoate formed in this reaction, the proportion of the 2E, 4E-ester falls as the temperature is reduced from +40 to −40‡C; the proportion of the 2E, 4E-ester in the heterophase system base/aprotic solvent increases with increasing polarity; and in the presence of 10 mole % of 18-crown-6 ether the proportion of the 2E, 4E-isomer is increased, being greater than 50% in all solvents. These findings indicate that the stereochemical outcome of the reaction depends on the balance between the phase systems and the chemical equilibria, including the formation, dissociation, and reactions of the intermediate products.