The purpose of this work is to determine experimentally the frequency shift of a quartz crystal resonator in contact with an inert gas, over a wide range of density of the gas, and to evaluate the various factors affecting this observed behavior. Four different gases were used (Ar, N-2, He, and H-2), allowing a wide range of densities to be studied, over a range of pressure of 0.5-50 atm. Three types of surfaces were used : gold, as obtained from the manufacturer of the quartz crystal; the same electroplated with a rough layer of nickel; the same nickel surface polished mechanically. Hydrogen was not studied in combination with nickel because absorption of the gas occurs spontaneously at room temperature. The total frequency shift observed can be divided into contributions due to added mass, to the pressure, and to the product of density and viscosity. Under the present experimental conditions the effect of added mass is negligible, since adsorption does not occur. The effects of pressure, density, and viscosity have been calculated from equations given in the literature. When all these are corrected for, there is a residual effect associated with the interaction of the fluid with a rough surface. A detailed analysis of the effect of roughness on frequency cannot be made, since the roughness is of a random nature. Theories developed for a number of limiting cases indicated that the frequency shift associated with surface roughness should be proportional to the density of the fluid. This relationship was found to hold experimentally. The roughness affecting the frequency of the quartz crystal resonator cannot be related in a simple manner to the surface roughness determined by standard electrochemical methods or by STM. Nevertheless, it will be possible to detect changes in surface morphology by their effect on the frequency in most cases.