Fourier transform ion cyclotron resonance mass spectrometry has been used to study the temperature and deuterium isotope effects on the methyl cation transfer reaction between protonated dimethyl ether and dimethyl ether to produce trimethyloxonium cation and methanol. From the temperature dependence of this bimolecular reaction it was possible to obtain thermodynamic information concerning the energy barrier for methyl cation transfer for the first time. From the slope of an Arrhenius plot, a value for DeltaH(double dagger) of -1.1 +/- 1.2 kJ mol(-1) was obtained, while from the intercept a value for DeltaS(double dagger) of -116 +/- 15 J K-1 mol(-1) was derived. This yields a DeltaG(double dagger)(298) value of 33.7 +/- 2.1 W mol(-1). All thermodynamic values were in good agreement with ab initio calculations. Rate constant ratios for the unimolecular dissociation forming trimethyloxonium cation rind the dissociation re-forming reactants were extracted from the apparent bimolecular rate constant. Attempts at modeling the temperature dependence and isotope effects of the unimolecular dissociation forming trimethyloxonium cation were also made.