Contact pairs of Ag, W, and Ag‐W were operated 6000 times in a 20‐A 110‐V ac circuit. The effect of contact‐resistance change was observed by measuring the temperature rise (TR) at the fixed contact while passing the steady‐state 20 A. TheTRwas measured before the switching had begun and after each 1000 operations. A large number of contact pairs were used in order to ensure a good statistical interpretation of the data. The chemical composition of the contact surfaces was measured using a Debye‐Scherer x‐ray analysis. The distribution of contact resistance for Ag contacts remained the same throughout the experiment. For W contacts, the distribution was initially higher than that for Ag: the result of conductivity and hardness differences. During the switching experiments, the formation of tungsten oxides on the contact surface resulted in high contact‐resistance values. For Ag‐W, the distribution was initially closer to that for Ag contacts: the result of a thin surface layer of Ag on the Ag‐W. During the switching, the Ag‐W contact‐resistance distribution gradually increased in dispersion and had higher values than those measured for W contacts. The x‐ray data showed that not only were tungsten oxides formed, but also there was a high probability that some of the free Ag on the contact surface formed an insulating silver tungstate.