Abstract: Purpose of the proposed research is to investigate the fundamental aspects of catalysis of the O2 reduction by metal-free porphyrins at the electrochemically polarized liquid-liquid interfaces, with a complementary insight gained from the study of catalysis at a solid electrode and in a homogeneous solution. Experimental electrochemical, spectroscopic and microscopic approaches will be combined with numeric simulations of electrochemical processes and density functional theory calculations of the reaction system. We intend to address the following issues: (a) mechanism and kinetics of the catalytic O2 reduction at the polarized interfaces, (b) role of the interfacial adsorption, (c) mechanism and kinetics of the catalytic O2 reduction in the homogeneous solution, and (d) quantum-chemical description of the O2 activation by metal-free porphyrins. In each of these issues, the attention will be paid to structural effects emerging from the substitution on the porphine ring. Results of this study should elucidate the principles leading to the O2 activation without using metal ions.