In this paper, we studied treatment of mixture of three azo dyes using homogeneous photoelectro-Fenton (PEF) combined with heterogeneous photocatalytic process (UV/TiO2). Carbon nanotube immobilized onto graphite plate (CNT/graphite) electrode was used as cathode. The investigated photocatalyst was TiO2 nanoparticles (Degussa P25) immobilized on glass plates. A rapid analytical methodology based on recording UV-Vis spectra during the photoelectro-Fenton/photocatalytic decolorization process and the data treatment using multivariate calibration methods such as partial least square (PLS) was developed. The three textile dyes (C.I. Acid Yellow 36 (AY36), C.I. Acid Red 14 (AR14) and C.I. Basic Yellow 28 (BY28)) were quantified simultaneously despite the overlap of their spectra. Also, the central composite design has been employed to the optimization of photoelectro-Fenton/photocatalytic decolorization of the dye solution containing three dyes using combined process. The factors (variables) investigated were the initial concentration of three dyes, initial amount of Fe3+ and reaction time. Results indicated that the optimum Fe3+ concentration was 0.15 mM for decolorization process. Analysis of variance (ANOVA) showed that predicted values match the experimental values reasonably well with R-2 of 0.965, 0.943 and 0.947 for AY36, AR14 and BY28, respectively. Three-dimensional surface plots have been used to present the relation between independent variables and dependent variable (response). (C) 2012 Elsevier B.V. All rights reserved.