The reason why H@sub 2@ desorption from Si(100) surface shows a first-order desorption kinetics has been a controversial issue for more than a decade, and various desorption models have been proposed accordingly. Although most of the models assume prepairing of surface hydrogen atoms at a dimer as a precursor state, few attempts have ever been made to confirm its role in the desorption kinetics. We here show that the reaction order for the H@sub 2@ desorption from Si(100) can be varied by changing the hydrogenating gas and the thermal condition of the hydrogenation and that its behavior is systematically interpreted as a change of fractional coverage of paired hydrogen atoms. Three hydrogenating gases (atomic hydrogen (H), silane, and disilane) and three thermal conditions (room-temperature adsorption (RT), high-temperature adsorption (HT), and post-annealing (PA)) were tested. The desorption kinetics was analyzed by the peak position, the spectral shape, and their coverage dependence of the temperature-programmed desorption (TPD) spectra. As a result, the desorption kinetic order increased as H < disilane < silane and RT < HT < PA. To investigate the microscopic detail, we developed a set of rate equations for desorption with a fractional coverage of unpaired hydrogen atoms being chosen as a key parameter, which described the whole variation of the TPD spectra quantitatively. Using the obtained parameter, we argue the dependence on the hydrogenating gas in terms of different arrangements of surface H atoms. The dependence on the thermal condition is explained by a selective desorption from paired hydrogen atoms as well as the dissociation of paired hydrogen atoms during thermal treatments.