This paper investigates the Fenton photo-assisted degradation of 4-chlorophenol (4-CP). The experimental work was planned according to the Design of Experiments (DOE) scheme, and degradation performance was measured in terms of TOC reduction at different reaction times. Appropriate reagent ratios have been obtained for effective 4-CP degradation. The modeling of the process performance as a function of the reaction conditions was then studied by fitting different models to the degradation data. The models are compared and discussed according to the degree of correlation they provide. The widely used second order polynomial equation (including bilinear terms) is questioned and alternative models are proposed (potential model, Hoed equation) for a better empirical representation of the performance of the degradation process. The disparate estimations given by the different models studied suggest that further research is required in model selection in order to obtain reliable characterization of Fenton, Fenton-like and photo-Fenton treatment performance. (C) 2008 Elsevier B.V. All rights reserved.