The competitive adsorption isotherms of two closely related peptides, bradykinin and kallidin, were measured by frontal analysis on a Zorbax SB-C18 microbore column. An aqueous soluton at 20% acetonitrile (0.1% TFA) was used as the mobile phase. The competitive isotherm data were fitted to four different models: Langmuir, Bilangmuir, Langmuir-Freundlich, and Toth. These data fitted best to a Bilangmuir isotherm model. The influence of the pressure on the retention factors of the two peptides was found to be small and was not investigated in detail. The band profiles of large samples of the single components and of their mixtures were recorded. The overloaded profiles calculated using either the equilibrium-dispersive or POR model are in excellent agreement with the experimental profiles in all cases. Our results confirm that the competitive isotherm data derived from mixtures may suffice for a reasonably accurate prediction of the band profiles of all mixtures of the two components, provided their composition is close to 1/1.