The pulsed thermal lens experiment is useful for investigating the kinetics and energetics of intermediate species in photoinitiated reactions. We have adapted this technique to allow decay rates to be estimated under conditions where second-order processes make a significant contribution to the observed kinetics. The excited triplet state photophysics of benzophenone in acetonitrile are investigated using this method. Using a mixed-order kinetic model, the rate constant of triplet-triplet annihilation was found to be (1.9 +/- 0.2) x 10(10) M(-1) s(-1), indistinguishable from a diffusion-controlled value. A finite rate of self-quenching of the excited triplet state was also found. From the amplitudes of the photothermal transients, the triplet energy of the benzophenone triplet was found to be 67 +/- 4 kcal/mol in acetonitrile. We also use this method to investigate the kinetics and energetics of hydrogen abstraction from 2-propanol, where the rate constant of radical production was found to be (4.5 +/- 0.3) X 10(6) M(-1) s(-1) and the O-H bond energy of the ketyl radical in acetonitrile was found to be 103 +/- 3 kcal/mol.