The rates and mechanisms of chemisorption on the Si(111)-(7×7) surface have been investigated under UHV conditions for SiH4, Si2H6, and Si3H8. Temperature programmed desorption of H2 has been used to quantitate adsorption of the silanes following calibrated exposures. A dramatic enhancement of the adsorption rate occurs with one SiSi bond in the molecule. A second SiSi bond has no effect on adsorption rate on the clean surface, but increases the reactivity with a hydrogen covered surface. The reactive sticking coefficient, SR, for silane is less than 0.001 near zero coverage on Si(111)-(7×7). SR (SiH4) is independent of temperature from 25 to 275°C. Reactive sticking coefficients of 0.47±0.1 for higher silanes Si2H6 and Si3H8 are observed at low coverage for 25°C surface temperature. The higher silanes adsorb on the clean surface through molecular precursor states, as evidenced by coverage independent sticking coefficients and by negative activation energies for adsorption. At low coverage, Si2D6 and Si2H6 exhibit the same sticking coefficient, so that no deuterium kinetic isotope effect is observed for the adsorption of disilane. This observation is consistent with SiSi bond breaking as the rate limiting step for conversion of the molecular precursor to chemisorbed species. The surface residence time of molecular SiH4 is inferred to be very short, relative to the residence times of molecular Si2H6 and Si3H8. SiH4 is relatively unreactive due to its short residence time and because adsorption requires SiH bond scission. © 1988.