The oxidation of asphalt is a major cause of pavement failure. The low-temperature oxidation kinetics of 14 asphalts are presented. At constant temperature and oxygen pressure, asphalt oxidation occurs in two stages : (1) a relatively rapid-rate period; followed by (2) a long period of constant rate. Activation energies for the constant-rate region vary from 64 to 109 KJ/mol, and reaction orders relative to oxygen pressure vary from 0.25 to 0.61. This variation in activation energy and reaction order leads to skepticism regarding the present practices of evaluating road-condition asphalt-hardening rates at a single elevated temperature and perhaps at an elevated pressure. The asphalts occur in essentially two groups, one at high values of both activation energy and reaction order and the other at low values of each. The data indicate the existence of art isokinetic temperature near 100 degrees C. The degree of oxidation that occurs during the initial rapid-rate region varies inversely with the oxygen reaction order of the constant-rate region.