The effect of Group 1 counterions on the decarboxylation rate of four carboxylate ions (trifluoroacetate, propiolate, cyanoacetate, and malonate) was determined in water at 140-280 degreesC and 275 bar. The decarboxylation kinetics were determined in real time by using IR spectroscopy and a titanium cell flow-reactor with sapphire windows. The complexity of the reaction of CF3CO2- necessitated additional postreaction studies by H-1 and F-19 NMR spectroscopy and F- ion electrochemistry. Batch mode reactions in a titanium tube reactor were employed for the latter studies. The CF3H product of CF3CO2- decomposed, liberating F- ions. The cation effect for CF3CO2- is caused primarily by the reaction of F- with the. Group I cations in the order Li+ > Na+ > K+..., which changed the ionic strength. The rate of decarboxylation of cyanoacetate was unaffected by the cation, whereas the Li+ ion slowed the rate of decarboxylation of propiolate, perhaps because of complexation with propiolate. These results were compared with the previously investigated malonate system. Where a cation effect was observed, the effect could be attributed to some form of ion pairing, but in each case the details of the interaction differ slightly.