The injection of samples whose viscosities are appreciably different from íhat of the motile phase can result in highly distorted elution profiles, showing severai maxima. The distortions are produced at the rear of the band when the viscosity of the sample solvent is higher than that of the motile phase. It is shown that the distortion of the peaks produced on injecting arbutin in columns of aminopropyl silica, using water as mobile phase and mixtures of 2-propanol and water as the sample solvent, grows in importance when the sample size increases, when the viscosity of the sample solvent increases and when the flow rate is decreased On the other side distortions at the band front are observed when arbutin dissolved in water or in acetonitrile + water mixtures is injected in a 2-propanol-water (50:50) mobile phase of higher viscosity. These trends are coincident with the predictions of a hydrodynamic instability criterion postulated several years ago and up to now almost entirely ignored in chromatography, except in relation with size exclusión chromatography.