Using a capillary viscometry technique, the high-shear-rate behavior of two polymer additives (an ethylene-propylene block copolymer and a radial hydrogenated styrene-isoprene copolymer) in a hydrocarbon-based oil solution has been investigated. At mass concentrations of up to 2.0% for the styrene-isoprene copolymer and 1.5% for the ethylene-propylene additive, the viscosity was measured over a range of shear rates from 10(4) to 10(6) s(-1). To correct for the effects of viscous heating and pressure changes, a numerical correction procedure is used which reduces the experimental results to viscosity data at a common reference temperature and pressure tor comparison. Over the range of shear rates examined, the styrene-isoprene solutions exhibited typical shear-thinning behavior, becoming more dramatic at higher polymer concentrations. In addition to shear thinning at the higher shear rates, a shear-thickening region was observed in the more concentrated ethylene-propylene solutions. As the polymer concentration increased, the degree of shear thickening was shown to be more severe and the critical region was observed at lower shear rates.