The adsorption of cationic tetradecyltrimethylammonium bromide (TTAB) from aqueous solution on two series of powdered MCM-41 aluminosilicates (referred to as SiAlxCn, where x is the mole Si:Al ratio and n the chain length of the surfactant template) has been studied at 298 K under the conditions of free pH of the aqueous phase. Potentiometric acid-base titrations of SiAl32C14 in NaNO3 and TTAB solutions were performed. The results of the previous study on the adsorption of ionic surfactants and the new proton-balance data indicate that the MCM-41 surface acquires a negative charge under the experimental conditions employed. The influence of the pore size and the surface charge density on the adsorption mechanism was evidenced by comparing adsorption isotherms of the surface-active ions TTA(+), electrophoretic mobilities of solid particles, and differential molar enthalpies of displacement obtained on various solid samples. For the hydrogen-exchanged SiAl32C14 sample (H+-SiAl32C14), the adsorption of bromide counterions Br- at the solid-solution interface and the isotherm of the pH evolution in the equilibrated supernatant liquid were additionally measured. Titration microcalorimetry was also used to determine the molar enthalpy of micellization and the critical micelle concentration of TTAB in deionized water at 298 K. Surfactant adsorption appears to occur simultaneously on the external surface area and the pore walls. Monolayered aggregates consisting of "head-on" oriented surfactant units are expected to form on hydrophilic, highly charged surface zones of Na+-derived samples at low bulk molalities: It is postulated that displacing and destructuring of vicinal water induced by the growth of compact aggregates are responsible for the positive enthalpy of displacement in this adsorption range. The endothermic domain broadens with decreasing pore size and increasing aluminum content in the silica matrix. The displacement process is exothermic over the whole experimental range on H+-SiAl32C14 because water structuring is not significant for this sample. The aggregates formed on solid samples at higher surface densities are very heterogeneous in size and shape, depending on the surface charge density and the pore space available to the adsorbate.