The pyrolysis of a trimethylsiloxylated silica (TMS-silica) in vacuo up to 750 degrees C gives a "reactive silica" with surface species and properties similar to those previously reported by degassing a methoxylated silica above 600 degrees C. Both surface silanes absorbing around 2290 cm(-1) and very stable Si-C drop CH groups appear. A dissociative chemisorption of water on the reactive silica at 25 degrees C, leading to both SiH and SiOH groups, involves silylene centers (SiO)(2)Si: with a bicoordinated silicon atom. A concentration of about 0.13 sorption centers nm(-2) is estimated, which is a value higher than on pyrolyzed ex-methoxylated silica. The SiH stretching range in the infrared spectra of TMS-silica displays a larger variety of intermediate SiH sites, at increasing temperatures, than for a methoxylated silica. In addition, the pyrolysis of TMS-silica creates intermediate aliphatic species absorbing at 2975 and 2986 cm(-1), correlated to the intensity decrease of the v(a)CH(3) band of the TMS group at 2964 cm(-1). Similar intermediate components, assigned to Si(CH3)(2) and SiCH3 groups, were observed by oxidation of TMS-silica in air, although at much lower temperatures than in vacuo. A SiCH3 bond is thus attributed to the previously unidentified species which absorbs at 2983 cm(-1) during the last stages of the pyrolysis of a methoxylated silica.