The difference between the vapor pressure of completely protiated or deuterated methane (CH4 or CD4) and partially deuterated methanes (CH3D, CH2D2, or CHD3) 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/T+B, where the prime always refers to the Lighter molecule. Within the studied temperature range, the vapor pressure isotope effect was found to be "inverse" (p >p’) for all the systems, except in the low-temperature range of the (CH4/CH3D) 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 CH4/CHD3) system, our vapor pressure isotope effect (VPIE) results were also compared with liquid-vapor isotope fractionation factor (LVIFF) data from other authors.