The aim of this study was to examine and determine binding affinities of the peripheral benzodiazepine receptor (PBR) with diamino acid derivatives of protoporphyrin IX with general structure PP(AA)(2) in a function of environmental pH changes. Fluorescence intensity was used as a measure of protein association constant. Decrease in protein fluorescence, after titration with increasing concentration of porphyrins, gave evidence of interactions between PBR and examined ligands. Experimental data were fitted to theoretical curves, assuming different models of interactions. All examined ligands were best fitted to weak binding model. Increase in environmental acidity resulted in changes in association constants. For all examined derivatives association constants were at least twice higher in pH 5.5 as compared to pH 7.4 and 6.5.