The effect of the properties of the silica gel matrix, and of the octadecylsilane used, on the chromatographic selectivity of C18 bonded phases is described. Preparation of bonded phases with monochlorodimethyloctadecylsilane is easier because absolute water exclusion is not required in this case. The surface coverage (μmol/m2) is smaller than with dichloromethyl- or trichlorooctadecylsilanes, but in these cases a secondary silanization with HMDS is essential to remove newly formed silanol groups. Bonded phases on Si 60 give the largest amount of hydrocarbon per unit column volume, large k′ values and good selectivity, however, surface coverage and efficiency of these packed columns is less than with Si 100 silica. The chemical properties of the silica surface also influence bonded phase selectivity. Bonded phases should not be compared with only a single eluent, because properties of bonded phases differ with test solutes and test conditions, i.e. eluent compositi