<p>Chlorophyll (Chl) <i>a</i> and 3,8-divinyl (DV)-Chl <i>a</i> are crucial pigments in efficient light-energy conversion in oxygenic photosynthesis. (C13²<i>S</i>)-Epimers of these pigments, Chl <i>a</i>′ and DV-Chl <i>a</i>′, form the primary electron donor in photosystem (PS)-I reaction center (RC) complexes. Despite the importance of Chl <i>a</i>′ and DV-Chl <i>a</i>′, no information is available on their in vivo formation. The substitution effects on epimerization reactions of Chl <i>a</i> and its analogs provide useful information to address this issue. In this study, the effects of vinyl groups in Chl pigments on epimerization properties were investigated using Chl <i>a</i>, DV-Chl <i>a</i>, and 3-devinyl-3-ethyl Chl <i>a</i> (mesoChl <i>a</i>). The vinyl group at the 8-position of DV-Chl <i>a</i> accelerated the epimerization reaction. This acceleration originates from the more electronegative C8-vinyl group in DV-Chl <i>a</i> compared to the C8-ethyl group in Chl <i>a</i>. By contrast, the transformation of the C3-ethyl to C3-vinyl group barely affects the epimerization kinetics of Chls. These results indicate that the vinyl group in the B-ring of the chlorin macrocycle has a larger effect on the epimerization of Chl <i>a</i> analogs than that in the A-ring. The thermodynamic properties of the epimerization of Chl <i>a</i> and its analogs were barely influenced by the vinyl groups at the 3- and 8-positions. The current results will provide a better understanding of the formation of the prime-type epimers of Chl <i>a</i> and DV-Chl <i>a</i> in PS-I RCs.</p>