Time-resolved thermal lensing (TRTL) techniques have been applied to resolve photophysical and thermodynamic parameters for various metastable species, of which the relaxation dynamics are dominated by the radiationless transition. Using 1H-phenalen-1-one as a sensitizer, the lifetime (τΔ), absolute quantum yield (ΦΔ) and efficiency of generation (STΔ) of singlet molecular oxygen (1Δg) have been determined in various organic solvents. In particular, through the development of an ultrasensitive detecting system, TRTL studies on the relaxation dynamics of O2(1Δg) become feasible in aqueous solution. In another application, based on the TRTL technique in combination with the O2 sensitization experiment, the T1-S0 energy gap of the proton-transfer tautomer in 3-hydroxyflavone, which is otherwise inaccessible due to the dominant radiationless deactivation, has been deduced to be ∼ 8500 ± 1020 cm−1.