The use of capillary zone electrophoresis to determine the adulteration of cows' milk in goats' milk products is described. The detection and quantification of cows' milk was based on the presence of the specific whey proteins: the relative calibration curve is reported. The peaks of interest were well resolved by using sodium borate at pH 9.2 as background electrolyte in methyl-silanized capillaries. The minimum amount detectable of cows' milk was 2% in milk mixtures and 4% in cheeses. Restrictions due to genetic variability and possible heat treatments, on only one of the two types of milk employed, are taken into account. Qualitative analysis of goat-ewe-cow and goat-ewe samples are also reported.