Abstract—As a model for studies involving biologic systems, rates of hematoporphyrin and protopor‐phyrin‐sensitized photooxidation of tryptophan were measured in water‐methanol mixtures. Absorption and fluorescence‐emission spectra associated with solutions of porphyrins in different water‐methano! mixtures were also examined. The optimal rate of photooxidation by hematoporphyrin occurred in 30% methanol; by protoporphyrin, in 60% methanol. Absorption spectra were examined to characterize dispersion of hematoporphyrin into monomeric form in 90% methanol, and the dimer formation which predominates in 30% methanol. With the more hydrophobic protoporphyrin, an absorption spectrum characteristic of drug monomer was seen in 100% methanol, a dimer spectrum was observed in 60% methanol. At methanol concentrations<60%, we observed impaired photooxidation and another alteration in the absorption pattern apparently reflecting drug aggregation beyond the dimer stage. Fluorescence emission maxima of both porphyrins were blue‐shifted with increasing solvent dielectric constant, but an additional blue shift was detected in an aqueous environment which was apparently related to porphyrin aggregation. Absorption and fluorescence‐emission spectra appear helpful for characterizing environmental determinants of porphyrin‐sensitiz