To compare the powers of the corresponding enzymes as catalysts, the rates of uncatalyzed decarboxylation of several aliphatic acids (oxalate, malonate, acetoacetate, and oxaloacetate) were determined at elevated temperatures and extrapolated to 25 degrees C. In the extreme case of oxalate, the rate of the uncatalyzed reaction at pH 4.2 was 1.1 x 10(-12) s(-1), implying a 2.5 x 10(13)-fold rate enhancement by oxalate decarboxylase. Whereas the enzymatic decarboxylation of oxalate requires O(2) and Mn(II) the uncatalyzed reaction is unaffected by the presence of these cofactors and appears to proceed by heterolytic elimination of CO(2).