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 CHiD4-i (with i = 0 or 4) and CHjD4-7 (with j=1, 2, or 3), the differential vapor pressure between mixtures of (CHiD4-i+CHjD4-j) and CH4 (if i=4) or CHjD4-j (if i=0) and the absolute vapor pressure of CH4 (if i=4) or CHjD4-j (if i=0), 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, GE(x,T) and the excess molar enthalpy HE(x), assuming that this last function is independent of temperature in the experimental range. The deviations from ideal behavior are very small, GE being only some tenths of J/mol for equimolar mixtures. The experimental GE 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 CD3H-CH4 system are also made.