In this work, in situ external infrared reflection absorption spectroscopy (IRRAS) is successfully employed for the detection of intermediate species in the oxygen reduction reaction (ORR) mechanism on a flat and well-defined Pt surface. Superoxide anion species (O2-) are detected on the Pt(111) surface in an O2-saturated solution with a NaF/HClO4 mixture with pH 5.5 by the observation of a O-O vibration band at ca. 1080 cm-1. The observation of O2- without the use of any other additional method of signal enhancement is possible because in these experimental conditions O2- is the main ORR-generated intermediate and its reactivity is limited in this pH. This leads to the accumulation of O2- near the Pt surface, facilitating its identification.