The photophysics of triplet excited alpha-terthiophene (alpha T) is investigated in the absence and presence of beta-cyclodextrin (beta-CD). Adding beta-CD to an aqueous alpha T solution causes the ground state terthiophene to form an inclusion complex with the cyclodextrin molecule. This host/guest complex has a 1:1 stoichiometry with an association constant K = 5850 +/- 230 M(-1). With no added beta-CD, the alpha T triplet in aqueous methanol decays with a decay time = 80 mu s. Adding beta-CD increases the decay time of the triplet to approximately 100 mu s. The decay in the compartmentalized system was first order, while that in the homogeneous solution was mixed first and second order. The second-order component is due to triplet-triplet annihilation. In homogeneous solution this annihilation leads to emission of delayed fluorescence. The compartmentalized systems also showed delayed fluoresence but with a lowered intensity as compared to the homogeneous systems with an equal amount of triplets formed. Compartmentalization does not shield the probe completely from triplet-triplet annihilation or other possible triplet deactivation paths. Tripler quenching by methyl viologen is diffusion controlled in homogeneous solutions but somewhat slower in beta-CD media. The rate of quenching of triplet alpha T by Cu(II) is less than diffusion controlled in noncompartmentalized systems. The rate constant of quenching is substantially lowered in the presence of the cyclodextrin. The reduction in quenching rate constant is attributed to compartmentalization in both cases. The quenching data indicate that the alpha T triplet is fully complexed by beta-CD.