The kinetics of decarboxylation of the alpha -substituted acrylic acid itaconic acid were analyzed at 280-330 degreesC, 275 bar, and solution pH(25) = 0.8-5.75 using an FTIR spectroscopy flow reactor. As the pH was increased, the observed rate constant initially decreased to a minimum at pH(25) = 1.82, then increased and reached its maximum at pH(25) = 3.75, and then decreased again up to the limit of study at pH(25) = 5.75. The decarboxylation rate depends on the molecular form of itaconic acid in the order of itaconate monoanion > protonated itaconic acid > neutral itaconic acid approximate to itaconate dianion. Insight into possible reasons for this trend was obtained from geometry-optimized structures using density functional theory with the B3LYP method at the 6-31+G* level.