Dielectric measurements have been applied successfully to obtain accurate values of the compressibility factor of methane in the presence of the small quantities of water vapor [20, 65, and 95 ppm(v)]. Up to about 95 ppm(v) of H2O in CH4, the compressibility factor decreases very slightly in the presence of H2O. The compressibility factor of dry CH4 determined by the dielectric method is compared with the best PVT results and the Goodwin equation of state. The compressibility factor of CH4 having small quantities of water vapor is compared with the Goodwin, GERG, and AGA8 equations.