Pretreatment methods of long-aged Pt electrodes are commonly used to obtain increased and reproducible electrocatalytic activities prior to electroanalytical measurement. However, many pretreatment methods can cause severe surface damage. Moreover, the degree and time dependence of the increased activities have rarely been investigated systematically. This paper reports a new pretreatment method (NaBH₄ treatment) of long-aged Pt electrodes and compares four pretreatment methods (NaBH₄ treatment, mechanical polishing, cathodic treatment, and anodic treatment) in terms of the degree and time dependence of the electrocatalytic activities toward oxygen reduction reaction. NaBH₄ treatment allows increased electrocatalytic activities comparable to those obtained after other pretreatments, without a surface damage. The high electrocatalytic activities of pretreated Pt electrodes decrease with aging in ambient air and in solution (0.1 M HClO₄); the activities decrease at a higher rate in air than in solution. The decrease of the electrocatalytic activities of Pt electrodes with aging is due to the increase of surface contamination.