A new reagent was used for the investigation of active oxygen in solids : 2,2,6,6 tetramethyl-1-hydroxypiperidine. It allows the determination of the oxygen chemisorbed on the surface of adsorbents and catalysts quantitatively. The behaviour of active oxygen in high-T-c superconductors presents a special interest, and this reagent has been used successfully in this case. From these experimental results the existence of molecular oxygen in these materials can be hypothesized.