The type II singlet molecular oxygen [O2((1)Δ g )]-mediated photo-oxidation of the tripeptide gly-tyr-ala was studied. It has two non-oxidizable amino-acids (gly and ala) bonded to the oxidizable one, tyr. Overall (k t) and reactive (k r) rate constants for the interaction were determined by time-resolved methods (IR emission of O2((1)Δ g )) and stationary photolysis, in water at pH 11.5 as well as in alkaline non-aqueous etOH-MeCN (80:20, v/v, 10 mM in KOH) solutions. An important solvent polarity effect onk t was detected; the rate constant increasing one order of magnitude in going from the organic mixture to water (k t H2O = 2 × 10(9) M(-1) s(-1)). Nevertheless,k r does not parallel this trend; gly-tyr-ala being less photooxidizable in a more polar environment. The effective quantum yield (∅ r ) forTPE photooxidation is much higher in etOH-MeCN (∅ r = 0.056) than in water (∅ r = 0.023). Results are discussed on the basis of the formation of an exciplex with polar character between the TPE and O2((1)Δ g ).Two remarkable points should be taken into account: a) the rate costants for the interaction of O2((1)Δ g ) with gly-tyr-ala are practically the same as for free tyr. b) New -NH2 groups are generated upon sensitized irradiation. Both findings indicate that the peptide bonds in the TPE break as a result of the photooxidation. A thorough analysis with data for tyrosine and related dipeptides is undertaken.