The aim of this investigation was to study the effect of molecular structure on the adsorption of cationic surfactants onto silica. Thus, the thermodynamics of adsorption of DTAB (dodecyltrimethylammonium bromide) and of the gemini surfactant 12-2-12 (ethanediyl-1,2-bis(dodecyldimethylammonium bromide)) on raw silica (SiNa) and HCl-washed silica (SiH) has been investigated mainly by the microcalorimetric technique. The determination of differential molar enthalpies of adsorption first requires accurate knowledge of the dilution enthalpy of each surfactant under the same experimental conditions as those used during adsorption. The values of the adsorption enthalpies, corrected by the dilution effect, give information on adsorption processes. Moreover, the amount of surfactant adsorbed, the sodium ion concentration, the pH in equilibrated supernatants, and the electrophoretic mobility data complete the calorimetric results. Analysis of the experimental results allows estimation of the number of the surface sites and building up, in both cases, a picture of the adsorption process. The different steps of the adsorption process (by electrostatic and hydrophobic interactions) are described, as are also the different shapes of aggregates at the end of the adsorption. These depend on the state of the surface and the surfactant. A high density of SiNa sites leads to a dense bilayer. A spread density of SiH sites gives rise to pinned micelles.