Novel cationic Ga(III)phthalocyanine (hydroxo[tetrakis(N,N,N-trimethylanilinium) phthalocyaninato] gallium(III), [Ga^<III>(TMAPc)(OH)]^<4+>) was synthesized and its adsorption and photochemical behavior on clay were investigated. Ga^<III>(TMAPc)(OH) adsorbed on the clay surface without an aggregation at high density conditions. The fluorescence quantum yield and excited lifetime of Ga^<III>(TMAPc)(OH)^<4+> on clay was 0.33 and 2.1 ns at 0.05% vs. CEC, respectively. Photo-oxygenation reaction sensitized by Ga^<III>(TMAPc)(OH)^<4+> on clay was examined. Cyclohexen-2-ol and 1,2-dichlorocyclohexane were obtained as major products. Because the Size-matching rule suppresses a molecular aggregation, Ga^<III>(TMAPc)(OH)^<4+> had photochemical activity on the clay surface.