The effects of different reaction parameters, such as catalyst, cocatalyst, and temperature, on reactivity ratios have been studied for the case of the copolymerization of propylene oxide (M1) with tetrahydrofuran (M2). The reactivity ratios were found to vary with the type of catalyst and cocatalyst used; however, the product of their values, r1r2, remained essentially constant. For the three catalysts studied, viz., 1) triphenylmethyl cations, (C6H5)3C+, in combination with three different gegenions (hexafluorophosphate, PF6−; hexafluoroarsenate, AsF6−; and hexafluoroantimonate, SbF6−; 2) antimony pentachloride, SbCl5; and 3) borontrifluoride etherate, BF3:(C2H5)2O, the values of r1, calculated on the basis of constant reactivity ratio products, were found to increase with temperature in the range −10 to 20°C. However, for BF3:(C2H5)2O initiated copolymerizations, the values of ri decreased between −30 and −10°C. The magnitude of this increase or decrease depends on the concentration of the diol cocatalyst used. The energetics of the propagation reactions studied, as obtained from plots of In r1and In r2vs 1/T, showed that the concentration of diol had practically no effect on the thermodynamic parameters, ΔH and ΔS. Based on activation enthalpy (ΔH11- ΔH12) and entropy (ΔS11- ΔS12) differences for increasing reactivity of PO, the catalysts were found to lie in the following order with respect to their relative reactivities: (C6H5)3C+SbF6−> (C6H5)3C+AsF6−> SbCl5> (C6H5)3C+PF6−> BF3:(C2H5)2O. The different activation enthalpy and entropy values obtained in the two temperature ranges mentioned above are attributed to the presence of different ion-pairs.