the invention relates to a method for deactivating and recovering boron trifluoride when producing polyisobutenes by means of cationic polymerization of isobutene or hydrocarbon streams containing isobutene in the liquid phase in the presence of boron trifluoride or in the form of a boron trifluoride catalyst complex . the catalyst complex is separated , essentially in the liquid phase , from the reactor discharge . the method comprises the following steps : a ) removing from the polymerization reactor at − 60 to 020 c ., methanol , ethanol or a mixture of methanol and ethanol in such a quantity that an alcohol phase rich in boron trifluoride is formed ; b ) separating the alcohol phase according to and , optionally recycling the boron trifluoride of the alcohol phase obtained from to the method in a suitable manner .