Preparative liquid column chromatography is a powerful method for the isolation of proteins from complex mixtures. The use of solvent gradients has proved invaluable in the optimisation of the chromatographic separation process. In contrast to isocratic chromatography, the eluting strength of the solvent is increased during the separation. To this end, an increasing proportion of a solvent of high eluting strength is added to solvent of low eluting strength. This publication is intended to contribute to the general understanding of and to a quantitative description of preparative gradient chromatography of dissolved proteins. The analysis is based on the experimental determination of various chromatograms of the model protein lysozyme on a cation exchanger.