A process is described for functionalizing isobutene polymers having terminal olefinic unsaturation by treating with the reaction product of a borane source with from 0.5 to 1.8 equivalents of an alkene having a molecular weight of less than 250 per mole of borane or by treating the isobutene polymer with the borane source in the presence of the alkene. The reaction mixtures retain low viscosity and good stirrability.