The effect of methylene blue (MB) encapsulation in cucurbit[7]uril (CB[7]) on triplet excited-state behavior and singlet oxygen (10,) generation has been studied by using laser flash photolysis (LFP) and time-resolved near-IR luminescence spectroscopy. The lifetime of the triplet excited state of M B is longer in the CB[7] cavity (140 mu s for MB-CB[7] vs 79.5 mu s for aqueous MB). Cucurbituril also protects the dye triplets from quenching by oxygen, reducing the quenching rate constant [k(q)(O-2)] from 2.6 x 10(9) M-1 s(-1) to 0.2 x 10(9) M-1 s(-1). The quantum yield of O-1(2) production in air-equilibrated D2O solutions is similar for free MB and for MB-CB[7], and the singlet oxygen lifetime is similar to 70 mu s, suggesting its decay occurs in the aqueous (D2O) phase. The generation of singlet oxygen is delayed by CB[7] this is attributed to the time required for oxygen to access the CB[7] nanocavity and react with the MB triplet. Thus, the rate-limiting step for sensitization is the entry of oxygen into the CB[7] cavity. Encapsulation inside CB[7] increases the relative efficiency of photoinduced MB2+center dot, dication-radical generation, for which a modest yield is observed.