The coupled kinetics of decarboxylation and hydrolysis of methyl propiolate were determined in real time at 160-210 degreesC and 275 bar by using a titanium-cell flow-reactor equipped with sapphire windows for direct IR spectral detection. The rate constants were calculated from the formation of CO2 and the disappearance of methyl propiolate at the natural pH of the solution. The product acid (propiolic) also decarboxylates, and its kinetics were determined at 150-200 degreesC and 275 bar for use in the overall model. At 180 degreesC, the relative rates are in the order propiolic acid decarboxylation > methyl propiolate decarboxylation > methyl propiolate hydrolysis, Hence, the hydrolysis step is rate-determining over most of the temperature range studied. Because of the instability of the propiolic acid product, the hydrolysis of methyl propiolate is not acid-catalyzed. Instead, the Arrhenius activation energy for the ester hydrolysis more closely resembles those previously reported for base catalysis.