The self-association behavior of propantheline bromide in a 154 mmol dm(-3) sodium bromide solution has been investigated by frontal chromatography on two Sephadex columns and by the surface tension method. Analysis of centroid volume data by asymptotic theory yields the monomer concentration of propantheline bromide as a function of its total concentration. Further analysis of the concentration dependence of the monomer concentration by a multiple equilibrium model provides weight-average and number-average aggregation numbers. The aggregates of propantheline bromide are small and are formed in a stepwise manner without a cmc. Stepwise aggregation constants for propantheline bromide are estimated and are used to predict the aggregate size distribution. Derivative chromatograms support the above results based on the centroid volume data. For instance, the shape of the derivative chromatogram of propantheline bromide at the trailing boundary is closer to that of penicillin V than to that of the surfactant, suggesting that propantheline bromide self-associates in a nonmicellar mode. The dimerization constant of propantheline bromide estimated from the trailing peak volume is close to that obtained from the centroid volume. The shape of the frontal chromatogram is almost independent of the size of the column. This fact indicates that our chromatographic method for investigating the self-association behavior is reliable. The surface tension of the propantheline bromide solution has a transition, albeit diffuse, when plotted against the logarithm of the total concentration and decreases gradually with increasing concentration above this transition. A reasonable molecular area of propantheline bromide on the aqueous surface is obtained by using the monomer concentration, determined by the chromatographic data, in the Gibbs adsorption equation.