A new upper limit to the adiabatic ionization energy of trans-hydroxyoxomethyl, EI(t-HOCO) less than or equal to 8.195 +/-0.022 eV, is provided, producing a lower limit to the enthalpy of formation, Delta H(f0)degrees(t-HOCO) greater than or equal to - 45.8 +/- 0.7 kcal/mol (greater than or equal to - 46.5 +/- 0.7 kcal/mol at 298 K). The spectrum shows progressions in C=O and C-O stretches of HOCO+ and provides evidence for the excitation of OCO bend. In addition, the data tentatively suggest an ionization onset as low as 8.06 +/- 0.03 eV. While it is not clear whether the latter corresponds to cis or trans isomer, it may indicate that Delta H(f0)degrees(HOCO) is even higher.