The retention behavior of low-concentration inorganic anions was studied systematically as a function of changing high matrix anion concentration. Stoichiometric retention model was developed for interpretation and prediction of matrix effects in ion chromatography. The model was tested and utilized for separation of low concentration (1-5 mg/l) bromate, bromide, chloride, nitrate, ethanesulfonate, and pentanesulfonate ions with carbonate buffer and hydroxide eluent in relatively wide ranges of sulfate matrix (>2000 mg/l). Equilibrium-based approach effectively characterizes the selectivity of the complex system through differences in ion-exchange constants of analyte, eluent and matrix ions. Selectivity data were determined from the experimental retention data by iterative calculations using the derived equations. The predicted and observed retention data are in rather good agreement. The method describes precisely the retention shift of trace anions in the high level of ionic matrices. The results in quantitative three-dimensional retention surfaces (k, matrix and eluent concentration) together with species distribution graphs are also presented.