We present results of a theoretical study on the photoinduced hydrogen atom transfer reaction in free-base porphin. Electronic structure calculations using density functional theory (DFT) with Becke's three-parameter hybrid functional are carried out to predict the structures and energetics for the low-lying triplet states of free-base porphin isomers. Transtion state searches were also carried out to locate the structures and barriers of the cis-trans and trans-trans transition states. Our results support the proposal that the trans-trans hydrogen transfer occurs on the tripler potential energy surface (PES) in a stepwise fashion through a cis isomer. After being excited from the ground state, the molecule is predicted to move into the trans T-2(B-3(1u)) State on the triplet PES and undergoes trans-cis isomerization, The cis-trans isomerization reaction is predicted to occur primarly on the (3)A' PES and produces the lowest triplet trans T-1(B-3(2u)).