SummaryUpon injection of concentrated polymer solutions in size exclusion chromatography, a random peak deformation has been observed and attributed to viscous fingering. In order to characterize this phenomenon, which is due to the difference in sample and eluent viscosities, mixtures of methanol and isopropanol are used as a low-molecular weight model. This system permits the study of viscosity effects in the absence of any adsorption or exclusion. Depending on the percentage of isopropanol added to a pure methanol mobile phase or to a methanol "sample" plug, a wide range of sample and eluent viscosities can be obtained. Statistical moments are used to compare elution profiles and to correlate peak deformation to viscosity differences. Significant loss of efficiency is observed whenever sample and eluent viscosity differ by more than 10 %.