Green River oil shale previously extracted by benzene was heated at 150, 200, 250, 300, and 350 degrees C for 0.5 to 360 days to determine the rate at which kerogen degrades to thermal products, the types of thermal products formed, and the effect of prolonged ;pw-temperature heating upon subsequent assay yields. Thermal decomposition of kerogen at 150 to 350 degrees C is treated as a first order reaction. The specific reaction rate for kerogen decomposition increases from 2.2 x 10 -4 per day at 150 degrees C to 2.0 x 10 -1 per day at 350 degrees C. The calculated kerogen activation energy is 19 kcal. Variations in the rate of formation of individual type components of the pyrolytic products were found. Carbon balances show an increase in the amount of kerogen converted to bitumen and oil as the temperature of preheating increased from 150 to 350 degrees C. Total product from shale preheated at temperatures of 150 and 200 degrees C gave less than 100 percent of Fischer assay yields while the total products gave greater than 100 percent of Fischer assay yields.