The aim of this work was to examine the performance of a vertical subsurface constructed wetland system in treating olive mill wastewater (OMW). The experimental pilot scale wetlands consisted of PVC reactors, some filling materials such as gravel, sand, and zeolite and the natural wetland vegetation including Typha latifolia and Cyperus alternatifolius. The filling materials in the wetland were stratified so that at the very bottom, the first layer was 17 cm of gravel, and on top there existed a 10 cm zeolite layer and the layer closest to the surface was a 7 cm sand layer. Concentration based average removal efficiencies for T. latifolia (W1), C. alternatifolius (W2), control reactors (W3) were COD; 73.46, 73.91, 71.40% NH4-N; 49.06, 37.38, 39.02% and PO4-P; 95.43, 95.93, 94.47% respectively. A statistically significant difference has been monitored between the wastewater feed and the reactor outlets, too, in terms of the color elimination. (C) 2010 Elsevier B.V. All rights reserved.