The proton affinity and absolute heat of formation of trifluoromethanol have been derived from translational energy threshold measurements for reactions involving oxygen-protonated trifluoromethanol. The reaction of ionized iodotrifluoromethane with water was used to prepare CF3OH2+ in the flow tube of a flowing afterglow triple-quadrupole instrument. The isomeric cluster ion, (HF)CF2OH+, was shown to be more stable than CF3OH2+ by the base-catalyzed conversion of CF3OH2+ to (HF)CF2OH+ using either SO2 or OCS as the catalyst. The proton affinity of CF3OH at oxygen was determined from the enthalpy change for the endothermic proton transfer reaction CF3OH2+ + CO --> CF3OH + HCO+. The measured enthalpy change, 9.2 +/- 1.4 kcal mol(-1), was combined with the known value for the proton affinity of CO (141.9 kcal mol(-1)) to yield a value for the oxygen proton affinity of CF3OH of 151.1 +/- 1.7 kcal mol(-1). The dissociation energy for the loss of water from CF3OH2+ was measured to be 36.6 +/- 2.1 kcal mol(-1) by energy-resolved collision-induced dissociation. This value was used in a thermochemical cycle along with the measured proton affinity of CF3OH to derive the gas-phase heat of formation of CF3OH of -220.7 +/- 3.2 kcal mol(-1). This experimental value is slightly lower than, but in good agreement with, the 298 K heat of formation of CF3OH that is predicted by high-level molecular orbital calculations.