The gas-phase decomposition kinetics of the title compounds have been examined over the temperature range of 369.9-439.4 degreesC and pressure range of 67.5-209.0 Torr. The reactions were found to be homogeneous and unimolecular and to follow a first-order rate law. The rate coefficients are expressed by the following Arrhenius equations: for methyl 2-hydroxypropionate (I), log k(1) (s(-1)) = (13.73 +/- 0.19) - (219.8 +/- 2.5) kJ(.)mol(-1) (2.303RT)(-1), and for methyl 2-hydroxyisobutyrate (II), log k(1) (s(-1)) = (13.37 +/- 0.39) - (217.1 +/- 5.1) kJ(.)mol(-1) (2.303RT)(-1). The decomposition products of I are acetaldehyde, methanol, and carbon monoxide, while II yields mainly acetone, methanol, and carbon monoxide and very small amounts of methyl formate and methyl methacrylate. The reaction mechanisms have been theoretically characterized at B3LYP/6-31+G**, MP2/6-31G**, and MP2/6-31++G** computing levels by characterizing the stationary points (reactants, products, intermediates, and transition structures) on the potential energy surface. The transition-state theory has been used to obtain the rate coefficients, and a good agreement is found between theoretical and experimental results.