To determine the quantitative relationship between the dye distribution coefficient and salt concentration during cationic dyeing, the author derived a modified Donnan-model based on hydrophobic and ionic interatctions between dyes and fibers. Analysis showed that the model accurately predicated the cationic dye equilibrium absorption on an acrylic fiber under different dyeing conditions. The model interpreted unusual phenomena, such as the concave shape of dye sorption isotherms with increasing sulfate and phosphate salt concentration. The model characterized the dual functions of simple electrolyes during cationic dyeing: the suppression of dye sorption through ionic interactions and the promotion of dye sorption through hydrophobic interactions. alothough the model fit most of the experimental data, the author considered that further refinements were necessary for the model to predict the dye sorption at high equilibrium concentrations in the presence of strong kosmotropes. 16 refs.