The effect of aqueous KC1 concentration on the electrical double layer properties of hexadecyltrimethylammonium chloride (CTAC) surfaces has been investigated. For self-assembled CTAC bilayers adsorbed onto amorphous silica surfaces, fiat plate streaming potentials ((s potentials), electrostatic potentials obtained from the electrophoretic mobility of colloid microspheres ((e potentials) and diffuse layer potentials (ffd) derived from the force versus separation curves for the interaction of a colloid microsphere with a fiat plate (measured with an atomic force microscope) have been analysed. Trends and differences in the (s and (, potentials have been attributed to incomplete electrical neutralisation of the silica surface by the inner adsorbed monolayer of CTAC, and differences in the silica surface charge density for the colloid and fiat plate. It is considered that derived ~'d values highlight the problem created by surface roughness for atomic force microscopy force curve analysis. CTAC micellar surface potentials 0Po), which have been determined by a solvatochromic acid-base indicator technique, are greater than the ( potentials for the planar adsorbed CTAC surfaces. Analyses in terms of a site-binding model and classical electrical double layer theory suggest that the degree of counterion dissociation from the surfactant headgroups (~) is significantly different for highly curved and planar surfaces; ~ is in the range 0.10-0.15 for the adsorbed CTAC bilayers and in the range 0.41-0.66 for spherical CTAC micelles when activities are used in the calculation, The CTAC critical micelle concentration and the minimum area per CTAC molecule at a planar interface as a function of KC1 concentration have been ascertained from air/aqueous solution surface tension curves. The hydrophobic contribution to the free energy of CTAC micellisation has been calculated on the basis that CTAC micelles behave as Nernstian objects in KC1 solution with the surfactant monomer acting as a potential determining ion. The results for the CTAC/KC1 systems have been compared with the results obtained for similar hexadecyltrimethylammonium bromide/KBr systems.