We present ab initio calculations of the heats of formation of (fluoroformyl)oxyl, FC(O)O, which is an intermediate in the stratospheric photo-oxidation of halocarbons, and fluoroformic acid, FC(O)OH. The G1 and G2 levels of theory are employed for both species. At 0 K G2 (G1) values are -88.0 (-87.2) kcal mol(-1) for FC(O)O and -148.0 (-146.6) for FC(O)OH. An isodesmic reaction yields a heat of formation of FC(O)OH of -146.9 +/- 3, the uncertainty arising largely from possible errors in the experimental data for other species in the reaction. The O-H bond energy of FC(O)OH is 111.6 (111.0) kcal mol(-1), about the same as that of the C-H bond in methane (similar to 110 kcal mol(-1)).