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5-Hydroxy-4-keto-2-pentenoic acid (HKPA) is characterized by the presence in its chemical structure of four different chemical groups (hydroxyl, keto, carboxylic and unsaturated groups) that render it a possible intermediate or monomer in the chemical industry. The production of this compound is studied by the sensitized photo-oxidation of 5-hydroxymethylfurfural (5-HMF) in aqueous solution at acidic pH. 5-HMF can be easily obtained through the degradation of saccharides such as fructose or inulin. The system reactivity is influenced by the concentration of methylene blue (BM) which is used as the photosensitizer and the oxygen dissolved in the solution. Only moderate consumptions of the sensitizer are recorded during the oxidation experiments thus suggesting the possibility to recover and reuse it in new runs. No differences in the reactivity of the system and/or selectivity for HKPA are observed as a result of changes in the starting concentration of the substrate and/or pH. A simplified reaction scheme of photo-oxidation of 5-HMF is proposed. Two kinetic models are developed according to this scheme to simulate the photo-oxidation processes of the substrate in the absence and in the presence of methylene blue. A validation of these models is carried out by means of the experimental data, collected at pH = 6.0, at different initial concentrations of 5-HMF and BM. © 2009 Elsevier B.V. All rights reserved.