Abstract: The adsorption and oxidation of 3,3′,5,5′-tetramethyl benzidine (TMB) on hectorite has been investigated using X-ray powder diffraction, ultraviolet-visible spectroscopy, electron spin resonance, and infrared spectroscopy. The molecule adsorbed by cation exchange at low adsorption levels and oxidized to the monomeric radical cation (yellow). At higher adsorption levels, intercalation of TMB occurred in amounts greater than the cation-exchange capacity of the hectorite, and the π-π charge-transfer complex (blue) became much more evident. The TMB monomers appeared to lie flat in the layer silicate interlayers, whereas the molecules in the charge-transfer complexes assumed a near-vertical orientation relative to the surface. The oxidation of adsorbed TMB was probably due to diffusion of O2 to the surface, because the structural Fe3+ content of the hectorite was too low to facilitate a significant quantity of direct Fe3+-TMB electron transfer.