This paper discusses the dependence of the etching properties of single-crystal silicon in tetramethyl ammonium hydroxide (TMAH) solutions containing small amounts of Triton X-100 surfactant on the preferential adsorption of the surfactant in relation to different surface orientations. In order to evaluate the orientation-dependent adsorption of the surfactant molecules on Si{1 1 0} and Si{1 0 0}, we use spectroscopic ellipsometry (SE) for the first time, finding that the surfactant is preferentially adsorbed on {1 1 0}. This is further confirmed by studying convex corner undercutting behavior. The dependence of the adsorbed surfactant thickness on various conditions, such as the surfactant bath time, temperature and ultrasonic agitation is considered. The dependence of the etch rate and the surface roughness in TMAH on the layer thickness shows that a pre-adsorbed surfactant layer affects these etching characteristics in a similar way as the direct addition of the surfactant to the etchant (TMAH + Triton), indirectly demonstrating that the surfactant is adsorbed at the interface during etching in TMAH + Triton. The preadsorbed surfactant layer produces a different evolution of the etching profile, showing potentially novel structures in MEMS.