In order to understand the adsorption and self-assembly of organosilanes, we have studied the adsorption of octyltrichlorosilane (OS) and octadecyltrichlorosilane (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 SiO. Heavier ions that carry information on the surface structure were also identified, such as CHSiO for an OS adsorption or CHSiO 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 AlOH 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 be 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.