Groundwater arsenic contamination has emerged as a major health threat to millions of people around the world. Studying the sorption process of As(III) and As(V) onto ferric hydroxide gel is important to understand the mobilization of arsenic under non-oxidizing conditions. Most of the previous adsorption studies were limited to single element or multi-element equilibrium in synthetic water. To investigate the effect of matrix and speciation in real groundwater systems, adsorption tests with added As(III) and As(V) separately and in mixture were conducted in both double-distilled deionized water (DDDW) and As-safe (<3 μg L-1) tubewell water. In DDDW, the As(III)/As(V) ratio in the mixture strongly influenced the sorption behaviour by shifting the adsorption edge and also the efficiency. For As(III) and As(V) mixture in 1:1 proportions in tubewell water, the adsorption of both the species decreased up to pH 8; in the alkaline ranges the adsorption extent was marginally increased in comparison to that in DDDW. When As(III):As(V) was added in 3:1 proportions in tubewell water, the adsorption of both was enhanced in alkaline ranges compared to that in DDDW. When As(V) was predominant [As(V):As(III) = 3:1] in tubewell water, no significant changes were observed for As(V), though adsorption of As(III) was notably retarded. The statistical analysis of the results indicates the concentration ratio of As species in the system has a definite impact on adsorption behaviour onto ferric hydroxide gel. Copyright © 2006, CAWQ.