Zinc tetrasulfo phthalocyanine (ZnPcS 4) was successfully doped into SiO 2 xerogel by sol gel technique and so a homogeneously doped composite was prepared. The changes of ZnPcS 4 UV/Vis absorption spectra during the sol gel process were studied correlated with the amount of dimers and monomers. The results show that the absorption intensity of ZnPcS 4 monomers increases with the time during the early stage of the sol gel process, but decreases in the latter stage. The reasons of such intensity variation are given from the change of the micro environment in which ZnPeS 4 existed during the sol gel process.