The kinetics of decarboxylation of 1.07 m malonic acid and monosodium malonate at 120-230 degrees C and 275 bar were determined directly in a combined microflow reactor and short-pathlength IR spectroscopy cell. Malonic acid and monosodium malonate decomposed in a single step to CO2 and acetic acid and CO2 and the acetate ion, respectively. The kinetics for malonic acid were modeled as an equilibrium between malonic acid and the monoanion with parallel decarboxylation of both species. The Arrhenius parameters are E(a) = 120 +/- 2 kJ/mol and In A (s(-1)) = 29.4 +/- 0.1 for the malonate monoanion and E(a) = 126 +/- 2 kJ/mol and In A (s(-1)) = 31.4 +/- 0.1 for malonic acid in a 316 stainless steel flow reactor at 120-210 degrees C. Decarboxylation of the monoanion is slower than that of malonic acid. The rates of decarboxylation of malonic acid are slower above 140 degrees C in a Pt/Ir alloy flow reactor with diamond windows than in a 316 stainless steel flow reactor with sapphire windows.