We report the first example of acceleration of a bimolecular reaction by increased solvent viscosity. In a series of hydrocarbon solvents of graded viscosity, the rates of two cycloadditions, the Diels-Alder dimerization of cyclopentadiene and the 1,3-dipolar cycloaddition of diphenyldiazomethane with ethyl phenylpropiolate, rise with increasing viscosity to about 1 cP and then fall thereafter. We interpret this rise as viscosity-induced acceleration up to similar to 1 cP, which cannot be accounted for by current kinetic theory. Several examples from the literature are reinterpreted in these terms, providing further support for our concept.