An experimental method is described for testing diffusion of gases in crude oils at pressures and temperatures up to 3,000 psia and 200 deg. F. A sampling technique was developed to withdraw by constant- pressure mercury displacement about 60 ul of oil from the diffusion cell through any of several 1/2-inch vertically spaced capillary tubes and to trap about 4 ul of the oil in a specially designed microsampler. Oil trapped in the microsampler can be completely inserted in the sample inlet of a chromatograph. From analysis in a three-stage chromatograph (GLC), gas concentration in a sample is determined from the ratio of gas peak heights to area of peaks of reference components (hexanes or heptanes) in the oil. A diffusion coefficient is calculated from measured concentration profiles with known equations for diffusion in finite or semi-infinte systems, depending on whether the gas has or has not reached the end of the oil column. Results are given for a test of methane diffusing at 2,560 psia and 160 deg. F. In crude oil from the weber sandstone reservoir in the rangely field, Colorado. A constant diffusion coefficient of 3.34 X 10-5 cm2 per second was obtained which gives a good fit between calculated and experimental profiles determined at six times after start of diffusion. The maximum standard error of gas concentration determinations experienced with this method was +- 0.88 Ml per ml of gas-free oil. Work done in cooperation with the University of Wyoming.