Co-Pt alloys were studied in detail by a corrosion test under phosphoric acid fuel cell (PAFC) conditions on the well-defined crystallographic structures for a typical combination of the alloy catalysts used in PAFCs, examining the long-life stabilities of the structures and the catalytic activities for O2 electroreduction. The ordered (O) and disordered (D) alloys at the same particle sizes can be obtained by heat-treating the mother alloy in different temperature sequences. The O-alloy exhibits a specific activity, an electrocatalytic activity based on the catalyst surface area, 1.35 times higher than the D-alloy before the corrosion test, but shows less activity (0.73 times) after the corrosion test, due to a higher degradation (47%) in the O-alloy activity as compared with that of the D-alloy (1%). It was found that the Co atoms on particle surfaces of both alloys dissolve easily in the acid. This is followed by a second slow dissolution from inside the alloy particles probably due to the protective action by a monolayer thickness of Pt remaining on the alloy surfaces, but the loss of Co in the second stage dissolution for the O-alloy is higher by several percentage points compared to that of the D-alloy. It was also found that the Pt content does not change on the catalyst support even after 50 h of corrosion test, but the pure Pt phase is formed in the corrosion product, where the phase for the O-alloy grows faster than that for the D-alloy with corrosion time. Based on these results, obtained by chemical, x-ray diffraction, and transmission electron microscopy with energy dispersive spectroscopy analyses, the corrosion for Pt alloy catalysts is clearly explained, i.e., after the dissolution of Co atoms in the first surface layer of alloys, both Co and Pt dissolve out simultaneously from small-size alloy particles and the Pt redeposits on the surfaces of large-size alloy particles (Ostwald ripening). It is concluded that the D-alloy is preferable to the O-alloy from the viewpoint of the stabilities in the structure and the electrocatalytic activity, differing from previous claims.