Novel phthalocyanine sulfonate ($H_2PPcS_x$) was synthesized from the sulfonation of a steric polymeric phthalocyanine and investigated as a photocatalyst in a homogenous system. The molecular weight of $H_2PPcS_x$ was characterized as 9213-11692 by MALDI-TOF MS, classified as an oligomer with a 14-18 degree of polymerization. The electronic spectrum indicated a less aggregative tendency of $H_2PPcS_x$ than the usual monomeric phthalocyanine photocatalyst. Efficient photodegradation of butyl Rhodamine B (bu Rh B) catalyzed by $H_2PPcS_x$ was achieved in a homogenous solution; the spectral character indicated that decomposition occurred on the benzene ring in the Rh B molecular. For a 50 mg $L^{-1}$ bu Rh B solution, the percentage degradation reached 95% in 4 h using 100 mg $H_2PPcS_x$ as a photocatalyst. A mechanism involving the participation of both singlet oxygen and radicals was established for the photodegradation.