Constructed wetlands are used for the treatment of wastewater containing metals. In order to clarify the role of plants, flow and the impact of organic matter, an investigation of three factors, each at two different levels, was carried out in small-scale model wetlands. The evaluated factors and levels were: type of flow (subsurface and surface); presence of plants (planted with Typha latifolia and unplanted) and addition of organic matter (with and without). Eight different experimental units were run for a year. The units were fed with synthetic wastewater containing chromium (VI) (1.5 mg L(-1)), zinc (1.5 mg L(-1)), macro, micronutrients and organic matter (to those units in which this factor was being investigated). Subsurface flow wetlands showed a significantly higher rate of chromium removal in comparison with surface flow systems (97 and 60 mg m(-2) d(-1), respectively). Planted systems removed significantly more chromium compared to unplanted systems (85 and 76 mg m(-2) d(-1), respectively), and the addition of organic matter increased the removal rate in a comparison with the units without it (88 and 69 mg m(-2) d(-1), respectively). Similar results were found for zinc; however, the addition of organic matter made no significant difference to zinc removal.