Recent studies in the literature show successful applications of Fe/alginate catalyst in heterogeneous Fenton reaction, taking advantage of the alginate property of forming solid gel structures when it is put in contact with divalent and trivalent cations. But little is done to study the contribution of homogeneous reaction on the overall efficiency of the process when working at circumneutral pHs. Fe(III)/alginate catalyst has been synthetized and tested for the degradation of sulfamethoxazole as model compound in two different matrices: deionized and bottled drinking water. Experiments were conducted by adjusting the initial pH to a wide range of values (2.0, 3.0, 5.5, 7.0, 9.0 and 11.0) in order to evaluate its effect over the performance of the photo-Fenton process. Experimental results revealed that the stability of Fe(III) coordinated with alginate strongly depends on media pH. It was confirmed that photo-Fenton processes conducted at pH over alginic acid pK(a) (4-4.2) are mainly heterogeneous, generating hydroxyl radicals on the surface of the catalyst but its effective reaction rate is very low, due to the scavenging effect of the catalyst on produced hydroxyl radicals. However, when the pH decreases below the alginate pK(a) due to the generation of acidic by-products, significant amounts of iron are released to the reaction media, turning the process into homogeneous with a minor heterogeneous contribution. A mechanistic explanation about this catalyst behavior which takes into account pH evolution along the reaction is proposed. (C) 2016 Elsevier B.V. All rights reserved.