Primary pΔJ(13C,H) and secondary sΔJ(13C,H) isotope effects on carbon-proton coupling constants were measured in studies of proton-and deuteron-coupled 13C NMR spectra of the halomethanes CHnDmX4-n-m, where X is Cl, Br or I. Both effects are always negative. The magnitudes of pΔJ(13C,H) increase with the number of halogen atoms whereas those of sΔJ(13C,H) decrease. pΔJ(13C,H) is almost linearly dependent on the number of halogen atoms whereas for sΔJ(13C,H) the dependence is non-linear. New results for the isotope effect on the 13C,H coupling constant in fluoroform are in accord with data for the other members of the haloform series. For CH3Br and CH3I estimates of the main features of the surfaces for the 13C,H coupling constants were made using known molecular force fields. © 1998 John Wiley & Sons, Ltd.