The influence of gamma-butyrolactone (gamma-BL), used as solvent of BF3-amine complexes, on the polymerization of monoepoxides was studied. Different intermediate reaction products of PGE initiated by BF3-4-methoxyaniline (BF3-4MA) previously solubilized in gamma-BL (55% by weight), were separated and analyzed by H-1-NMR. It is shown that there is the opening of both epoxy and gamma-BL. The latter does not homopolymerize, but copolymerizes well with epoxy groups. The use of a large quantity of gamma-BL leads to a decreasing molar mass of the formed polymer. The kinetic study allowed to propose the mechanism of the cationic polymerization of epoxy initiated by BF3-amine complex in the presence of gamma-BL.