The quenching rate constants of singlet oxygen by C-60, H-2@C-60, D-2@C-60, H-2, and D-2 in solution were measured. The presence of a hydrogen (H-2@C-60) or deuterium (D-2@C-60) molecule inside the fullerene did not produce any observable effect based on triplet lifetime or EPR measurements. However, a remarkable effect was found for the O-1(2) quenching by C-60, H-2@C-60, D-2@C-60, H-2, and D-2. Singlet oxygen was generated by photosensitization or by thermal decomposition of naphthalene endoperoxide derivatives. Comparison of the rate constants for quenching Of O-1(2) by H-2@C-60 and D-2@C-60 demonstrates a significant vibrational interaction between oxygen and H-2 inside the fullerene. The quenching rate constant for H-2 is 1 order of magnitude higher than that of D2, in agreement with the results observed for the quenching Of O-1(2) with H-2@C-60 or D-2@C-60.