The adsorption behavior of tetrabutylammonium (TBA) and cetylpyridinium (CP) cations was studied on Hypersil 5-ODS sorbent. After establishing that CP has about 5-fold stronger affinity to the C18surface than the structurally different, symmetric, spherical molecule of TBA, the adsorption from their common solution was also examined. The data of “competitive binding” seem to show that TBA should have certain selective binding sites on the C18surface from which it can not be displaced by the CP, even in the case where the latter is present in a 4-fold excess in the mobile phase.