AbstractRates of hydrolysis of the vinyl ether functional groups of (Z)‐ and (E)‐β‐methoxyacrylic acid and (Z)‐and (E)‐β‐methoxymethacrylic acid and their methyl esters were measured in aqueous perchloric acid solution. Additional rate measurements were also made for one substrate, (Z)‐β‐methoxymethacrylic acid, in buffer solutions down to pH 7, and a rate profile was constructed. The results show that the β‐carboxy and β‐carbomethoxy substituents produce strong rate retardations, ranging from 2000‐ to 25 000‐fold, for bothZ‐ andE‐isomers in both the acrylic and methacrylic acid series. The rate profile for (Z)‐β‐methoxymethacrylic acid indicates that ionization of this substrate to the carboxylate ion form rises the rate of hydrolysis by a factor of 240. It is argued that this difference in reactivity of ionized and non‐ionized forms of the substrate is due to conjugative and inductive effects of the substituents, rather than β‐lactone formation as suggested in an earlier observation of