Atomic equivalent (AEQ), BAC-MP4, G2(MP2), G2, CBS-4, CBS-Q, and CBS-QCI/APNO molecular orbital calculations were used to calculate enthalpies of formation in the series of fluoromethanes, CHxF4-x, x = 0-4. While the computed Delta(f)H degrees(BAC-MP4) and Delta(f)H degrees(CBS-4) were in close agreement with experiment, errors in enthalpies from the other five methods were relatively high. In particular, enthalpies of formation calculated with the G2(MP2) and G2 procedures exhibited systematic deviations from experiment which were linearly dependent upon the number of C-F bonds in the molecule. Application of isodesmic reaction calculations yielded values of Delta(f)H degrees(G2(MP2)), Delta(f)H degrees(G2), Delta(f)H degrees(CBS-Q), and Delta(f)H degrees(CBS-QCI/APNO) that were in remarkably close agreement with experiment. This technique had no significant effect on the quality of results from the AEQ, BAC-MP4, and CBS-4 methods.