The difference between the vapor pressure of completely protiated or deuterated methane CH 4 or CD 4 and partially deuterated methanes CH 3 D, CH 2 D 2 , or CHD 3 has been measured over the 96–121 K temperature range. The vapor pressure data obtained were fitted to equations of the type T ln(p/p)A/TB, where the prime always refers to the lighter molecule. Within the studied temperature range, the vapor pressure isotope effect was found to be ''inverse'' (pp) for all the systems, except in the low-temperature range of the (CH 4 /CH 3 D) system. Our data agrees with other results found in the literature, both experimental and theoretical. Differences in the enthalpy of vaporization were calculated from the experimental results. In the case of the (CH 4 /CHD 3) system, our vapor pressure isotope effect VPIE results were also compared with liquid–vapor isotope fractionation factor LVIFF data from other authors. © 1997 American Institute of Physics. S0021-96069750521-6