The reactivity ratios r1 (EO) and r2 (PO) for the copolymerization of ethylene oxide (EO) and propylene oxide (PO) have been determined. The reactions were carried out in dry dimethyl formamide at 90°C using the potassium salt of ethylene glycol as initiator. The polymer composition at different EO/PO ratios in the feed was determined by 1H-NMR. Using the Fineman-Ross method to calculate reactivity ratios from the values of polymer composition, rl- and r2-values of 1.8 and 0.3, respectively, were obtained.