Photo-excited Zn-phthalocyanine (ZnPc) interactions with four (hydro)peroxides (ROOR') were studied by T-T absorption decay and fluorescence quenching. Triplet state interactions are weak (log k(q) similar to 4) and independent from the quencher used, while singlet quenching is variable (log k(q) 4.5-9.4). EPR and spin-trapping show that at low ZnPc concentrations the primary decomposition products are alkoxyl radicals. At high ZnPc concentrations, photosensitization of ROOH gives rise to peroxyl radicals. The (hydro)peroxide reducing potentials were estimated following AMI MO-computations. Our experimental data and theoretical predictions (Marcus-Saveant theory) are consistent with a charge transfer mechanism to account for the alkoxyl radical formation. (C) 2003 Elsevier Science B.V. All rights reserved.