Transient absorption microspectroscopy was developed for studies on photophysical and photochemical processes in a single laser-manipulated microdroplet in water. Transient absorption spectra and their decay curves of individual tri-n-butyl phosphate (TBP) microdroplets containing zinc tetraphenylporphyrin (ZnTPP) were measured and analyzed. Owing to the intense laser excitation, the concentration of the ZnTPP triplet state was so high that the decay profiles of Tn ← T1 absorbance of ZnTPP obeyed the second-order kinetics, triplet-triplet (T-T) annihilation. Simultaneously the delayed fluorescence of ZnTPP was observed and analyzed in terms of T-T annihilation. The estimated rate constant of T-T annihilation (kTT) was comparable to that obtained from Tn ← T1 absorption spectroscopy. Assuming the T-T annihilation is diffusion-controlled, the inner viscosity of a single TBP microdroplet was obtained and examined as a function of the diameter of the microdroplets. A remarkable micrometer size effect that the inner viscosity is decreased with the droplet diameter was found and discussed in terms of a swollen TBP droplet owing to the penetration of water.