Nanostructured carbons were fluorinated and used as supports for Pt nanoparticles deposition using a modified polyol route. The resulting materials exhibited similar Pt nanoparticles sizes, but different agglomeration rates. The electrocatalysts were tested toward the oxygen reduction reaction, and their stability was investigated in simulated load cycling (0.6 < E < 1.0 V vs. RHE) or start-up/shutdown (1.0 < E < 1.5 V vs. RHE) protocols. Irrespective the support material, the former protocol caused Pt nanocrystallites dissolution/redeposition via Ostwald ripening, mildly decreasing the electrochemically-active surface area and ORR activity. In contrast, the carbon supports were strongly corroded after the start-up/shutdown protocol, resulting in pronounced detachment/agglomeration of Pt nanocrystallites, albeit in absence of significant particle-size growth. Fluorination had different effects on the stability of structurally-ordered and structurally-disordered carbons: beneficial effects were observed for the latter whereas the former was affected negatively. “Free” dangling groups present in structurally-disordered carbon, known to be prone to preferential oxidation in PEMFC environment, combine with the fluorine precursors upon fluorination, leading to formation of more robust C-F bonds versus oxidation than original C-O bonds. In contrast, fluorination of structurally-ordered carbon creates structural disorder (C-C bonds are broken), leading to promotion of electrochemical corrosion.