The reactivity of the well-defined surface organometallic fragment drop Si-O-Sn(n-C4H9)(3) 1 grafted on silica(200) and on silica(500) has been studied by thermal treatment of 1 at increasing temperatures in vacuo. The surface reactions have been followed by quantitative measurements of the evolved gases, infrared and Mossbauer spectroscopies, C-13 CP-MAS and Sn-119 NMR spectroscopy, XPS measurements, and electron microscopy (CTEM and STEM EDAX). On both types of silicas, the surface reactions are similar in nature, although differences are noticeable. First, there is formation of (drop Si-O)(2)(Sn(n-C4H9)(2)) 2, which undergoes a second solvolysis process by silanols leading to (drop Si-O)(3)Sn(n-C4H9) 3 and finally surface Sn(II) and Sn(IV) atoms (as determined by XPS and Mossbauer experiments). Although the well-defined surface organometallic compound (drop Si-O)(2)Sn(n-C4H9)(2) can be prepared on silica by another route, no unique surface compound can be obtained during the thermal decomposition which transforms progressively 1 into 2 and 3. A mechanism of decomposition of the various surface organometallic complexes has been deduced from a comparison of the results obtained on both solids. The alkyl groups seem to follow a beta-H elimination mechanism leading to tin hydrides and 1-butene rather than a disproportionation mechanism leading to equimolar amounts of 1-butene and butane.