The aggregation of hexadecyltrimethylammonium bromide, CTAB, on silica at pH 9.6 +/- 0.4 has been shown, using optical reflectometry and AFM, to occur at a significantly lower concentration than is seen at neutral pH. This lower critical surface aggregation concentration, CSAC, results from the increased surface charge of the silica. Although this concept is not new, AFM has provided the first direct evidence of lateral organization of adsorbed surfactant at an order of magnitude below the cmc. This confirms that a threshold surface concentration of adsorbed surfactant is required to initiate surface aggregation. Specifically, on silica an adsorbed density of 0.6 mg/m(2) is sufficient for hydrophobic interactions between neighboring adsorbates. This promotes adsorption beyond that required to balance the surface charge, and hence the formation of surface aggregates. In addition, slow adsorption is observed in the vicinity of the CSAC as reported recently at neutral pH. The results confirm that surface aggregation is not a simple function of the bulk surfactant concentration, but rather a complex function of surface charge characteristics in the case of oppositely charged surfactants adsorbed to silica.