Malonic acid (HO2CCH2CO2H) and its anions malonate(-1) (HO2CCH2CO2-) and malonate(-2) (-O2CCH2CO2-) are among the more abundant dicarboxylates in natural water. Two variables influencing their behavior are the pH and the counterion. The rate constants for decarboxylation were determined in the pH range of 1.89-7.0 with a flow reactor-FTIR spectroscopy cell operating at 140-240 degreesC and 275 bar. The relative rates of the first-order reactions are malonic acid > malonate(-1) much greater than malonate(-2). The Arrhenius activation energies are similar for the three species (116-120 kJ/mol), making the rate differences controlled primarily by the (-1) = 30.2, 28.3, and 23.3, respectively. These findings are interpreted in pre-exponential factors In(A, s(-1) terms of the role of entropy in a cyclic intermediate common to all three species. Malonate(-2) is proposed to form this structure by hydration to 1-orthomalonate(-2). The entropy decreases as the negative charge increases due to increased rigidity of the cyclic intermediate and increased electrostriction of the solvation shell. The influence of the Group 1 and 2 cations Li+, Na+, K+, Rb+, Cs+, Mg2+, Ca2+, and Sr2+ on the decarboxylation rate of malonate(-1) was determined. Except for Mg2+, the rate decreases with increasing ionic potential lion charge/ion radius), which is consistent with increasing replacement of H by the metal ion in the six-member cyclic intermediate. Mg2+ is anomalous possibly because it forms the strongest complex with malonate((-1)) and decarboxylates by a different mechanism.