An electrocoagulation (EC) process to remove fluoride and arsenic from groundwater (fluoride 5.5 mg L-1, arsenic 50.4 mu g L-1, hydrated silica 132 mg L-1, sulfate 40 mg L-1, nitrate 6.7 mg L-1, phosphate 0.55 mg L-1, hardness 23 mg L-1, alkalinity 59.0 mg L-1, pH 8.5 and conductivity 824.5 mu S cm(-1)) was investigated. The EC was carried out in a filter-press flow reactor, containing a three-cell stack equipped with aluminum electrodes. The influence of current density (j), mean linear flow rate in the EC reactor (u(r)) and the co-existing ions on the fluoride and arsenic removal efficiencies was analyzed. All EC tests, performed at 0.23 <= u(r) <= 0.93 cm s(-1) and 5 <= j <= 7 mA cm(-2), satisfied the World Health Organization (WHO) standard for fluoride (C-F- <= 1.5 mg L-1). The EC tests that satisfied the WHO standard for arsenic (C-As <= 10 mu g L-1) were at 0.23 cm s(-1) and 6 <= j <= 7 mA cm(-2). Spectroscopic analyses on aluminum flocs showed that these are mainly composed of aluminum silicates. Fluoride replaces a hydroxyl group from aluminum flocs and arsenates adsorb on aluminum aggregates. The best EC test in terms of energy consumption (6.7 kWh m(-3)) was obtained at 0.23 cm s(-1) and 6 mA cm(-2).