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Reactivity towards p-methoxybenzaldehyde (ArCHO) of electrochemically generated phosphonate carbanions has been investigated. Electrolyses were carried out at a mercury cathode in DMF and two routes to the desired carbanion have been compared: (i) Deprotonation of phosphonates of general formula (EtO)2P(O)CHYW (Y = W = Cl; Y = H, W = Cl; Y = Cl, W = CO2Et; Y = H, W = CO2Et; Y = CH3, W = CO2Et; Y = Cl, W = CH3), by the bases resulting from the electroreduction of azobenzene; addition of the carbanion formed onto the carbonyl group takes place and leads to the adduct (EtO)2P(O)CYW(Ar)O−. (ii) Two-electron reduction of halophosphonates (EtO)2P(O)CXYW (X = Cl, Y and W as above; X = Br, W = CO2Et, Y = Cl, Br, or CH3); when no H atom is present on the carbon bearing the phosphonate group (Y and W ≠ H), the same evolution leading to the above adduct is observed; on the contrary, when Y = H, the electrogenerated carbanion deprotonates the substrate and the resulting carbanion reacts with the aldehyde, giving the adduct(EtO)2P(O)CXW(Ar)O−.Evolution of the intermediate adduct depends on the substituants Y (or X) and W: when W = CO2Et, whatever the nature of Y (or X), diethyl phosphate is eliminated with formation of the ethylenic ArCH = CWY (or X) (Wittig–Horner reaction); the same evolution is observed when Y = W = Cl. When W = Cl and Y = H or CH3, the final product is the phosphonate epoxyde resulting from chloride elimination (Darzens reaction).Chemo- and stereoselectivity depend only on the nature of Y and W but are independent of the mode of generation of the carbanion. Yields are limited by side-protonation reactions, which are related to the basicity of the phosphonate carbanions. Analysis of the results permits selection of the optimal electrolysis conditions for purposes of synthesis. Keywords: electrosynthesis, electrogenerated bases, phosphonates, Wittig–Horner.