We have investigated the growth of Si from disilane on Si(001) in GSMBE. The growth rates were determined from reflection high energy electron diffraction (RHEED) intensity oscillations. Close inspection of the oscillation traces revealed a rapid decrease in intensity after beginning of growth for the first half period. This decrease was always faster than half the period of the remaining oscillation trace. The decrease does not depend on growth temperature but does depend on the incident flux. To interpret this effect, we assume that the surface is initially hydrogen-free. Under these circumstances, a maximum amount of disilane will quickly adsorb and decompose on the Si surface, which accounts for the sharp decrease in the beginning for high incident fluxes. Further adsorption (and thus regular growth) is only possible after subsequent desorption of hydrogen. Thus, in this time, the hydrogen surface coverage develops towards the equilibrium value, which is constant during the subsequent growth. It is assumed that during the time to reach the equilibrium (τ i), 0.5 ML of Si are deposited, which in turn corresponds to a minimum in RHEED intensity. Thus, by injecting a certain flux and measuring τ i, the determination of the reactive sticking coefficient α r, which is defined as the ratio between the amount of chemisorbing flux and the incoming flux on a bare surface, is possible. This way, the sticking coefficient of disilane on the Si(001) surface was found to be independent of substrate temperature in the investigated range between 464 and 580°C.