The gas-phase elimination kinetics of the title compounds have been examined over the temperature range of 310-369 degrees C and pressure range of 50-130 Torr. The reactions, in seasoned vessels, are homogeneous, unimolecular, and follow a first-order rate law. The products are methanol and the corresponding methyl ketene acetal. The rate coefficients are expressed by the Arrhenius equation: for trimethyl orthoacetate, log k(1) (s(-1)) = [(13.58 +/- 0.10) - (194.7 +/- 1.2) (kJ mol(-1))](2.303R7)(-1)r = 0.9998; and for trimethyl orthobutyrate, log k(1)(s(-1)) = [(13.97 +/- 0.37) - (195.3 +/- 1.6) (kJ mol(-1))](2.303RT)(-1)r = 0.9997. These reactions are believed to proceed through a polar concerted four-membered cyclic transition state type of mechanism.