The kinetics and pathways of decarboxylation of aqueous acetylenedicarboxylic acid at pH = 0.97-8.02 were studied in situ at 80-160 degreesC and 275 bar by using an FT-IR spectroscopy flow reactor with sapphire windows. The first-order (or pseudo first-order) rate constants and corresponding Arrhenius parameters were obtained for the neutral acid, monoanion, and dianion. The decarboxylation rates are in the order: HO(2)CCequivalent toCCO(2)(-) > HO(2)CCequivalent toCCO(2)H > (-)O(2)CCequivalent toCCO(2)(-). The decarboxylation mechanisms of these reactants and the propiolic acid product were analyzed by B3LYP/6-31+G(d) density functional theory. The transition state structures were found for the neutral acids and monoanions. In gas phase the transition state structure is a four-member ring involving C-C(O)-O-H. In aqueous solution a cyclic structure incorporating at least one water molecule forms. A comparison of transition state structures for the decarboxylation of beta-saturated (succinic) and unsaturated (maleic, fumaric, and acetylenedicarboxylic) aliphatic diacids was made with and without incorporating a water molecule. Consistent with experiment, the calculated activation energy for H-atom transfer to the alpha carbon atom in the decarboxylation step follows the order Cequivalent toC < Cequivalent toC < C-C.