Silica surface functionalization is mandatory for a large variety of applications. This step is often done through the condensation of functional silanes on silanol, silica surfaces' terminal groups. Among these silanes, aminopropyltriethoxysilane (APTES) is widely used due to its assumed high reactivity with silanols, kinetically promoted by the catalytic action of the terminal amine function. Here, we revisit, based on a quantitative analysis by solid state 29 Si NMR, the assembly of this silane on silica surface to investigate whether its presence result from grafting, i.e. hetero-condensation with silanol groups or from homo-condensation of silane molecules in solution leading to polycondensates physisorbed on the surface. We investigate the interaction of APTES with a crystalline layered silicate, the ilerite and with amorphous non-porous silica. We also studied the assembly on these materials of a second silane, cyanopropyltrichlorosilane (CPTCS), which exhibit a similar reactivity towards silanol groups but without the input of a terminal amine as it ends with a nitrile group. Unlike the classical characterization techniques, solid state 29 Si NMR, when used in quantitative conditions allows for the discrimination of grafted silanes from those homo-condensed in solution and physisorbed on the surface. Our results undoubtably prove that while CPTCS is grafted on the silica surface, the presence of APTES on silica and silicate materials is only marginally associated to silanols consumption. The analysis of the signal related to silicon atoms from silanes (T n species) and those from silica (Q n species) allowed for the accurate estimation of the extent of homo-condensation vs grafting based on the ratio TO -T/Q-O-T 2 siloxanes bridges. These findings deeply question the well-established certainties on APTES assembly on silica that should no longer be seen as grafting of alkoxysilane by hetero-condensation with silanol groups but more accurately as a homo-condensed network of silanes, predominantly physisorbed on the surface but including some sparse anchoring points to the surface involving less than 6 % of the overall silanols groups.