The electron paramagnetic resonance (EPR) spin probe 4-[N,N-dimethyl-N-(n-hexadecyl)ammonium]-2,2,6,6-tetramethylpiperindinyl-N′-oxyl (HTAB*) has been used to study the adsorption and aggregation of cetylpyridinium chloride (CPC) and cetylpyridinium salicylate (CPSa) on silica particles. In CPC micelles, the local viscosity, as determined from the rotational correlation times of HTAB*, was found to be approximately constant, as was the local polarity determined from Aiso, giso, and HTAB* equilibrium constants. In CPSa micelles at concentrations above the sphere-to-rod transition, significant decreases in polarity and local viscosity were observed. The affinity of CPC for the strong binding site on the silica surface was found to be higher than the affinities of CPSa and HTAB. The polarities and local viscosities of the surfactant aggregates and the corresponding micelles were found to be similar. From analysis of the line widths of the EPR spectra, it was concluded that CPSa on silica surfaces forms two coexisting aggregate phases, one which excludes HTAB* and one in which HTAB* is concentrated. The HTAB* is believed to be excluded from the former phase because of ordering of the salicylate counterion and pyridinium headgroups as proposed by Favoriti and Treiner. 1