Dealloyed Pt-Cu alloy nanoparticles are active oxygen reduction electrocatalysts; they are formed from Cu-rich alloy precursors during a selective Cu atom dissolution (dealloying) process. The surface of Cu-rich particle precursors is prone to oxidation under ambient air conditions, which may critically affect the aging behavior of the precursors. Here, we present a systematic stability and aging study of a carbon-supported Pt25Cu75 alloy nanoparticle catalyst precursor. We study the impact of the aging of the catalyst material on its electrocatalytic performance for the oxygen reduction reaction (ORR) after dealloying. We obtain a practical insight into the electrochemical behavior of the materials in the formats of powders, inks, and films. Our studies suggest that the Pt-Cu precursors show a stable catalytic performance when aged as dry powders in air. Ink samples, however, reach their maximum ORR activity of up to 1.3 A/mg(Pt) with aging for 24-48 h after which they deteriorated in performance. Finally, catalyst thin films were the most sensitive to aging in air and generally deteriorated rapidly after just one day. Our results provide practical insights and guidelines regarding the stability and handling of the nanoparticle catalyst powder.