Adsorption of the spin-probe 4-[N,N-dimethyl-N-(n-hexadecyl)ammonium] N＇-oxyl bromide (HTAB*) onto silica in aqueous dispersions has been studied using electron paramagnetic resonance (EPR) spectroscopy. The spin probe has been found to partition between three environments, the aqueous solution, the surface of the silica, and aggregates on the surface. The partitioning between these three environments is reported as a function of available silica surface, pH, and added hexadecyl-trimethylammonium bromide (HTAB). At low pH and large silica surface-area the spin probe appears to adsorb flat onto hydrophobic areas of the silica surface. At high pH and small surface area the spin probe forms surfactant aggregates.