Accurate heats of formation have been computed for ClFn (n = 1-3). The accuracy of the results is strongly dependent on the basis set quality and it is crucial to add at least one tight d function to Cl. The atomization energies are corrected for scalar relativistic, core-valence, spin-orbit, thermal effects, and zero-point energy. Our heats of formations are in very good agreement with the experimental values reported by Gurvich therefore suggesting that the JANAF values are slightly underestimated.