Threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy has been used to investigate the unimolecular chemistry of gas-phase, energy-selected methyl formate ions, CH3OCHO.+. The 0 K dissociation onsets for CO loss from CH3OCHO.+ and CH3OCDO.+ were 11.550 +/- 0.020 and 11.582 +/- 0.020 eV, respectively. The two-component dissociation rates are interpreted in terms of competitive fast dissociation and isomerization steps, followed by a slow dissociation from the lower energy isomer, a distonic ion with the structure CH2OCHOH.+. Ab initio molecular orbital calculations (G2 theory) are used to obtain energies and vibrational frequencies of various ion structures and transition states. The experimental rate constants are then modeled with the RRKM statistical theory for dissociation of both CH3OCHO.+ and CH2OCHOH.+ From the RRKM analysis, we found that the distonic ion CH2OCHOH.+ lies 16.2 +/- 1.5 kcal/mol lower than the methyl formate ion. Rearrangement of CH3OCHO.+ to its distonic isomer requires 15.2 +/- 1.0 kcal/mol.