Green River oil shale previously extracted by benzene was heated at 150 deg, 200 deg, 250 deg, 300 deg, and 350 deg 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 low-temperature heating (preheating) upon subsequent assay yields. Thermal decomposition of kerogen at 150 deg to 350 deg 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 deg c to 2.0 X 10-1 per day at 350 deg 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 (from assay of preheated oil shale) as the temperature of preheating increased from 150 deg to 350 deg c. The total product from shale preheated at temperatures of 150 deg and 200 deg c gave less than 100 percent of fischer assay yields while the total products from shale preheated at temperatures of 250 deg to 350 deg c gave greater than 100 percent of fischer assay yields.