Evidence for the self-aggregation of 10(S)-chlorophyll a ( = chlorophyll a′) in nonpolar solvents has been obtained by means of absorption spectroscopy in the visible and infrared regions. The visible absorption spectrum of chlorophyll a′ exhibits a shoulder at 680 nm in aliphatic hydrocarbon solvents and the infrared spectrum measured in carbon tetrachloride contains an “aggregation peak” at 1650 cm-1. IR spectroscopy shows, in addition, that the C-10 epimers of chlorophyll a are both present as monomers in tetrahydrofuran. However, in the presence of a small amount of aliphatic alcohol or water, the IR spectrum in tetrahydrofuran at lowered temperatures exhibits an intense, wide absorption band at ～ 1650 cm-1 which was interpreted as arising from the hydrogen bonding of the hydroxyl group to the C-9 keto carbonyl of the chlorophyll