Abstract The radiolytic degradation of 4-hydroxybenzoic acid ethyl ester (4-HBAEE) and 4-hydroxybenzoate (4-HBA) and the subsequent product formation in N2O-saturated and aerated solutions has been studied as a function of dose. The rate constants of OH• radicals with the substrates are k(OH•+4-HBAEE)=7.5×109 dm3 mol−1 s−1 and k(OH• +4-HBA)=6.7×109 dm3 mol−1 s−1. Irradiation of 5×10−4 mol dm−3 aqueous solutions (N2O, pH 6.0) of 4-HBA leads to the products 3,4-dihydroxybenzoic acid (3,4-DHBA) and hydroquinone (HQ). In the case of 4-HBAEE neither hydroxylation nor decarboxylation products are observable. The predominating reaction pathway with 4-HBAEE is water elimination from the primarily formed dihydroxycyclohexadienyl radicals. By pulse radiolysis a protonation equilibrium of these transients with pK=8.0 could be determined. The protonated OH•-adducts (λmax=385 nm, e=300 m2 mol−1) decay with k=7×104 s−1, the radical anions (λmax=425 nm, e=240 m2 mol−1) with k=4×105 s−1, yielding phenoxyl radicals λmax 405 nm, e=160 m2 mol−1 and 425 nm, e=175 m2 mol−1, 2k=3.6×108 dm3 mol−1 s−1, which do not form phenolic compounds. With 4-HBA OH•-adducts water splitting at pH 6 is very slow, k=4×103 s−1, therefore second order decay reactions can compete. At pH 10, where base catalysed water elimination takes place, no hydroxylation products are observable either. In aerated solutions dihydroxy-compounds are formed with both substrates, due to the fast addition of oxygen to the OH•-adducts. In the case of 4-HBA 68% of the OH• radicals result in 3,4-dihydroxyderivatives; for 4-HBAEE these are only 25%. The decarboxylation product hydroquinone is generated only from 4-HBA, its yield corresponds to 18% of the OH• radicals. Comparison of the initial degradation yields demonstrates 4-HBAEE to be 1.6 times more stable towards radiation, for a 50% decomposition of the ester a 2.3 times higher dose as for 4-HBA is required. The low rate of hydroxylation, the lack of decarboxylation products and the remarkable resistance to radiation of the ester of 4-hydroxybenzoic acid is of importance in respect to chemical changes in irradiated fruits and vegetables.