An instrument based on the Burnett method for determining gas compressibility at high temperature and high pressure was constructed. An estimate of the accumulated measurement errors indicate the data should be accurate to eight parts in 10,000. The instrument was calibrated up to 1,000 lb./sq.in. with helium at 200° and 300°C. The average absolute difference between the experimental and the smoothed helium compressibility factor data was 4 and 6 parts/100,000 for the 200° and 300°C. isotherms, respectively. Compressibility factor data for methyl chloride from 200° to 350°C. at 25°C. intervals and at pressures to 5,000 lb./sq.in. are reported. Second virial coefficients for helium and methyl chloride were also obtained.