In order to understand the adsorption and self-assembly of organosilanes, we have studied the adsorption of octyltrichlorosilane (OS) and ocradecyltrichlorosilane (OTS) on an aluminum surface, naturally oxidized. The self-assembled monolayer (SAM) formation was followed with the time-of-flight-secondary ion mass spectrometry (ToF-SIMS) technique. The adsorption is readily detected by means of the Si-containing ions, such as SiH+, SiH-, SIOH ' or SiO2-. Heavier ions that carry information on the surface structure were also identified, such as C8H17SiO2- for an OS adsorption or C18H37SiO2- for an OTS adsorption, The adsorption kinetics can be easily followed by monitoring several characteristic fragments as a function of the adsorption time. The OS adsorption on aluminum appears to have an oscillatory behavior, with a fast adsorption during the first minute followed by a desorption, then a slow adsorption again. All the Si-containing peaks reflect this trend. A possible model that explains the oscillation is given. The aluminum oxide fragments of the type AlxOyHz- exhibit a dramatic decrease with increased coverage, due to the reaction of the active surface sites with the: silanols. Interestingly. the OTS adsorption appears to he monotonic, with a coverage slowly increasing with time. The final achieved coverage is more complete with OTS than with OS. The ToF-SIMS technique appears to be well suited to study the chemical changes and the degree of surface coverage during the adsorption.