The initial adsorption of alkyltrimethylammonium bromides (C(n)TAB, n = 10, 12, and 14) on silica has been determined as a function of alkyl chain length at high solution pH. The isotherms showed a strong chain length dependence at low adsorption densities. The influence of the identity and valency of the surfactant co-ion on the initial adsorption was investigated by determination of the isotherms in 10 mmol.dm(-3) Br- solutions containing Na+, Ca2+, and C(1)TA(+) (that resemble the surfactant head group). The dependence of the adsorption on ionic strength and alkyl chain length is interpreted in terms of formation of small surfactant aggregates in the solution region adjacent to the charged oxide surface. A thermodynamic model, based on the Poisson-Boltzmann equation, that accounts for the formation of surfactant oligomers and the electrostatic binding of these aggregates in the diffuse layer has been developed. A qualitative agreement between calculated and experimental isotherms was found if only electrostatic interaction between the surface and the surfactant aggregates was considered.