This paper reports new results on the adsorption of cationic dimeric surfactants (12-s-12 surfactants, where s is the carbon number of the polymethylene spacer) on adsorbents with different surface functions, namely raw and HCl-treated silica. These results were obtained by traditional methods (adsorption isotherms, electrophoretic mobility, and chemical analysis of the equilibrated supernatant) and microcalorimetry. The results showed that the stoichiometry of the first step of the adsorption (ion-exchange step) varies strongly with the spacer carbon number. The binding of one surfactant to the surface brings about the release of between 1 and 2 sodium ions as the spacer carbon number is increased from 2 to 10. Thus the surfactant binding to the surface involves one head group for the 12-2-12 surfactant (short spacer) and two head groups for the 12-10-12 surfactant (long spacer). These results suggest the use of dimeric surfactants as molecular rulers to study the distribution of charged sites on surfaces. The microcalorimetric experiments clearly showed the two adsorption steps. The ion-exchange step gives rise to an endothermal effect having an amplitude that depends strongly on the spacer carbon number. The second adsorption step associated with the formation of surfactant aggregates gives rise to an exothermal effect that also depends on s.