The α-subunit of phycoerythrocyanin (α-PEC) can exist in four states (Z-α I , Z-α I I ,E -α I , E-α I I ). They are connected pairwise by photoreversible photochromism. The type I photochemistry connecting Z-α I and E-α I , involves aI5ZE phototransformation. α-PEC showing this type of photochemistry is obtained when the subunits of PEC are separated by gel permeation chromatography in the presence of 63 mM formic acid, or by reduction of the α-subunit of phycoerythrocyanin of type II reversible photochemistry with mercaptoethanol. α-PEC showing the recently characterized [Hong et al. (1993) Photochem. Photobiol. 58, 745-747] type II photochemistry connecting Z-α I I and E-α I I can be obtained when the α-subunit of phycoerythrocyanin of type I photochemistry is allowed to oxidize, or when it is treated with p-chloromercuribenzenesulfonate. The two types of reversible photochemistry of α-subunit of phycocoerythrocyanin are therefore controlled by the state of the two sulfhydryl group(s), viz. Cys-98,99 of the apoprotein. A quantitative analysis of the PCMS titration showed that modification of either one of these two cysteine residues is sufficient to inhibit type I photochemistry and induces type II. By treatment with mercaptoethanol or PCMS, the end products of type I and type II photochemistry, respectively, could be pairwise transformed into each other, showing that type II also involvesI5ZE isomerization. The difference between them must be due to different interactions between phycoviolobilin and apoprotein, which can be modulated by the two sulfhydryls.