Differential thermal curves of selected Colorado oil-shale samples showed endothermic dips in temperature range of 450 to 475 degrees C. These characteristic dips occurred within the temperature range of the conversion of organic material (kerogen) in oil shale to oil, gas, and organic residue. However, it is not known wither the dips resulted from heat absorbed in the vaporization of reaction products of possibly from endothermic reaction of kerogen. The thermal curves also showed the presence of calcite and/or dolomite but did not indicate the presence of clays.