The vapor pressure isotope effect of samples of isotopically substituted methane and their mixtures was measured as a function of temperature and mixture composition: The differential vapor pressure between CH i D 4i with i0 or 4 and CH j D 4 j with j1, 2, or 3, the differential vapor pressure between mixtures of (CH i D 4i CH j D 4 j) and CH 4 if i4 or CH j D 4 j if i0 and the absolute vapor pressure of CH 4 if i4 or CH j D 4 j if i0, were measured simultaneously between 96 and 121 K for mixtures of nominal composition 0.25, 0.50, and 0.75 mole fraction in the reference methane species. The p(x,T) data were used to calculate the excess molar Gibbs energy function, G E (x,T) and the excess molar enthalpy H E (x), assuming that this last function is independent of temperature in the experimental range. The deviations from ideal behavior are very small, G E being only some tenths of J/mol for equimolar mixtures. The experimental G E values compare well with estimated results based on a modified version of the statistical theory of isotope effects in condensed phases. Comparisons with reported values of the liquid–vapor isotope fractionation factor for the CD 3 H–CH 4 system are also made. © 1997 American Institute of Physics. S0021-96069750621-0