The adsorption and reaction of the amino acid glycine (NH2-CH2-COOH) are studied experimentally on the polar single crystal surface of zinc oxide, ZnO(000-1), by X-ray photoelectron spectroscopy (XPS) under UV light in presence and absence of molecular O2. Deposition at 350 K mainly resulted in a largely deprotonatedmonolayer (NH2-CH2-COO−(a)+OH(s); where O is surface oxygen,(a)is for adsorbed and(s)is for surface species) identified by its XPS C1s binding energy at 289.3 eV (-COO), 286.7 eV (-CH2-) and XPS O1s at 531.8 eV(-COO). A decrease in the signals of all functional groups of the adsorbed glycine (monitored by their C1s, O1s,and N1s lines) is seen upon UV excitation in the absence and presence of O2pressures up to 5 × 10−6 mbar. The photoreaction cross sections extracted from the decrease in the C1s peaks were found to be =2.6 × 10−18(COO(a)) and 1.4 × 10−18(-CH2-)cm^2. The photoactivity of the ZnO(000-1) surface under UHV-conditions is found to be comparable to that seen in direct photolysis of amino acids in solution.