The possibility of olive mill wastewaters (OMW) treatment and concentration by membrane distillation has been investigated. The objective is to obtain pure water and a concentrate containing high amounts of polyphenols, which can be extracted later. Two types of commercial membranes, polytetrafluoroethylene (TF200) and polyvinilydene fluoride (GVHP), were tested and the effects of membrane parameters on direct contact membrane distillation (DCMD) performance (i.e. permeate flux and polyphenols retention) were studied. The effects of different DCMD operating parameters were also investigated. The obtained results demonstrated the efficiency of DCMD for OMW treatment and concentration. The membrane TF200 showed a better separation coefficient (99%) after 9 h of DCMD operation than the membrane GVHP (89%) and the corresponding OMW concentration factor was found to be 1.72 for the membrane TF200, whereas it was only 1.4 for the GVHP membrane after 9 h operating time.