Direct measurement of the kinetics of kinesin dissociation from microtubules, the release of phosphate and ADP from kinesin, and rebinding of kinesin to the microtubule have defined the mechanism for the kinesin ATPase cycle. The processivity of ATP hydrolysis is ten molecules per site at low salt concentration but is reduced to one ATP per site at higher salt concentration. Kinesin dissociates from the microtubule after ATP hydrolysis. This step is rate-limiting. The subsequent rebinding of kinesin-ADP to the microtubule is fast, so kinesin spends only a small fraction of its duty cycle in the dissociated state. These results provide an explanation for the motility differences between skeletal myosin and kinesin.