Gaseous hydrogen atoms H(g) produced in hydrogen plasma abstract hydrogen adatoms H(ad) to generate hydrogen molecular desorption from Si surfaces. Direct abstraction, H(g) + D(ad) → HD(g), as well as indirect abstraction, H(g) + D(ad) → D₂(g), are found to show complex kinetics of desorption. The mechanisms of the direct and indirect abstraction paths are discussed in terms of both hot complex of H atoms bound to a HSi-SiH cell and thermodynamical instability of the dihydrides phase. Dynamics of the abstracted HD molecules is found to be much richer in translational energy in comparison with the case of thermal desorption. Based on the knowledge accumulated in the thermal desorption and abstraction experiments, hydrogen uptake as well as both the direct and indirect abstractions on Si(100) are simulated to reproduce the corresponding experimental data. Finally, the significance of hydrogen-surface chemistry in the plasma-enhanced Si CVD is discussed.