The excited triplet state deactivation of zinc(II) meso-diaryloctaalkylporphyrins (ZnDAOAP) has been studied over a wide temperature range using transient triplet-triplet absorption spectroscopy together with steady-state and time-resolved phosphorescence techniques, The results from transient absorption measurements show that the depopulation of the initially formed triplet state (T-1A state) is unusually fast at temperatures above 150 K. The efficiency of the deactivation originates from a spin allowed transition to a second tripler state (T-1B state), The transformation process T-1A-->T-1B is therefore the dominating deactivation channel of the T-1A state in this temperature range, and direct intersystem crossing T-1A-->S-0 makes negligible contribution. The subsequent ground-state recovery T-1B-->S-0 is also very efficient in comparison to many other porphyrins. Due to the substantial activation energy found for the transformation process, it most likely involves a conformational distortion of the porphyrin macrocycle. At low temperature, however, the relaxation of the T-1A State occurs by direct intersystem crossing to the ground state.