Composites of polytetrafluoroethylene (PTFE) and alpha phase alumina produce wear rates that can be nearly five orders of magnitude less than the wear rates of virgin PTFE. The mechanism for this reduction in wear cannot be explained solely by mechanical effects. The influences of oxygen and humidity on the tribological performance of both unfilled PTFE and PTFE/alumina composites were studied. The wear rate of PTFE/alumina composites is dependent on the humidity of the environment; this dependence suggests a tribochemical mechanism is responsible for the ultra-low wear behavior of these PTFE/alumina composites.