Positron annihilation spectroscopy (PALS) and differential scanning calorimetry (DSC) measurements were performed in pure and plasticized poly(ethylene oxide) (PEO)-dioctyl phthalate (DOP) and (PEO)-ethylene carbonate (EC) systems. The plasticized PEO samples were prepared by co-dissolution and melting methods in the concentration range of 0-25 wt% of plasticizer. The mean free volume hole size (V-f) values were calculated from the positron lifetime data. For the PEO-DOP it was observed an increase of the V-f values with increasing DOP concentration, while for PEO-EC system the V-f values remained constant. The relative concentration of vacancies showed to be not dependent on DOP concentration and for PEO-EC system it was observed a small decrease with the increase of EC concentration. The thermal behaviour characterized by DSC measurements showed variations of transition and melting temperatures as well as on the degree of crystallinity of PEO with plasticizer concentration. The PALS and DSC results clearly showed the presence of plasticization effect with DOP in opposite to the antiplasticization with EC. The compatibility limit of DOP in PEO was about 15 wt% in the preparation conditions of these samples. The efficiency of plasticization was estimated from the slope of V-f curves for PEO-DOP systems. (C) 2001 Published by Elsevier Science Ltd.