The simultaneous photocatalytic oxidation of As(Ill) and humic acid (HA) in aqueous Degussa P25 TiO2 suspensions was investigated. Preliminary photocatalytic studies of the binary As(III)/TiO2 and HA/TiO2 systems showed that As(III)was oxidized more rapidly than HA and the extent of photocatalytic oxidation of each individual component (i.e. As(Ill) or HA) increased with decreasing its initial concentration and/or increasing catalyst loading. The simultaneous photocatalytic oxidation of As(Ill) and HA in the ternary As(III)/HA/TiO2 system showed that both As(Ill) and HA oxidation was reduced in the ternary system compared to the corresponding binary systems. The effect of operating conditions in the ternary system, such as initial As(III), HA and TiO2 concentrations (in the range 3-20 mg/L, 10-100 mg/L and 50-250 mg/L respectively), initial solution pH (3.6-6.7) and reaction time (10-30 min), on photocatalytic As(III) and HA oxidation was assessed implementing a two-level factorial experimental design methodology. Seven and ten factors were found statistically important in the case of photocatalytic As(Ill) and HA oxidation respectively. Based on these statistically significant factors, a first order polynomial model describing As(III) and HA photocatalytic oxidation was constructed and a very good agreement was obtained between the experimental values and those predicted by the model, while the observed differences may be readily explained as random noise. (C) 2009 Elsevier B.V. All rights reserved.