We have studied a new microwave method to measure the compressibility factor and the water content of humid gaseous samples. This method uses a resonant cavity to measure the dielectric permittivity of the gas filling it. That measurement permits us to deduce the quantity of interest from the pressure and the dielectric constant virial expansions. By comparison of our measured compressibility factor of methane with the values computed from different equations of state, we have found that the microwave method is at least 0.1% accurate. We can also detect water vapor in methane down to a concentration of 75 ppm at 2 MPa.