Threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy has been used to investigate the unimolecular chemistry of gas-phase methyl propionate ions. This ion undergoes isomerization to a lower energy enol structure, CH3CHC(OH)OCH3.+, via two different pathways involving two distonic isomers, . CH2CH2C+(OH)OCH3 and CH3CH2C+(OH)DCH2.. This isomerization reaction is in competition with the direct CH3O. loss reaction, which leads to two-component dissociation rates of the methyl propionate ions. Detailed modeling of this complex reaction permitted the extraction of the dissociative photoionization threshold for methyl propionate, which at 0 K is 10.83 +/- 0.01 eV, as well as the isomerization barrier between the distonic CH3CH2C+(OH)OCH2. and enol CH3CHC(OH)OCH3.+ ions of 8.5 kcal/mol (relative to the methyl propionate ion). By combining this with the 0 K heats of formation of methyl propionate and the methoxy radical, we derive a 0 K heat of formation of the product propanoyl ion of 147 kcal/mol. Also measured was the adiabatic ionization energy of methyl propionate, 10.03 +/- 0.05 eV.