Measurement of coking rate under isothermal conditions has been conducted for a variety of test conditions. Variables included time, fuel flow rate, tube diameter, and surface roughness. Coking rate increases with temperature and Reynolds number, but is not dependent on surface roughness. The activation energy is approximately 10,000 cal/mole. Examination of deposits by infrared spectroscopy and the electron microprobe indicates that oxygen is present in large concentration, both singly- and doubly-bonded to carbon; in addition, nitrogen may be present in an imide structure. The aromatic content of the deposit is higher than that of the unstressed fuel. Transmission electron microscopy indicates the presence of agglomerates of particles about 15 Å in diameter. This suggests that insoluble particles originate within the liquid, although the low overall activation energy and the dependence of coking rate on Reynolds number suggest that oxidation of the fuel is mass-transfer-dependent and surface-catalyzed. Gas chromatographic and infrared analysis of polar fractions of stressed and unstressed fuel are virtually identical, indicating that deposit precursors exist at very low levels.