Two different types of chlorophyll c (c1 and c2) and their oxidized forms (c'1 and c'2) were isolated from a brown seaweed, Undaria pinnatifida, and their structures were detd. by mass spectrometry and 1H NMR spectroscopy. The order of conversion efficiencies in titania-based solar cells using these chlorophyll sensitizers, i.e., c2(4.6%) > c1(3.4%) > c'2(2.6%) c'1(2.5%), were explained in terms of a more extended conjugated system and the resultant higher electron densities in c2 than c1 as well as the different configurations and electron densities of the vinyl carboxyl group in c'2 and c'1.