Laboratory experiments were performed to determine the maximum operating temperature for cyclopentane as an organic Rankine cycle working fluid. The thermochemical decomposition of cyclopentane was measured in a recirculation loop at 240, 300, and 350 degrees C at 43 bar in a glass-lined heated tube. It was determined that, in the absence of air at the two lower temperatures, decomposition was minor after more than 12 days of continuous operation. At 240 degrees C, the total cyclopentane decomposition products were approximately 65 ppm, and at 300 degrees C, the total decomposition products were on the order of 270 ppm at the end of the experiment. At 350 degrees C, the decomposition products were significantly higher and reached 1500 ppm. When the feed was saturated with air under prevailing atmospheric conditions, the decomposition rate increased dramatically. Residues found in the reactor after the decomposition experiments were examined by a number of different techniques. The mass of the residues increased with experimental temperature but was lower at the same temperature when the feed was saturated with air. Analysis of the residues suggested that the residues were primarily heavy saturated hydrocarbons.