The adsorption of atoms and molecules of several gases of the Si(100)2×1 silicon reconstructed surface is investigated by surface differential reflectance spectroscopy. This UV-visible optical spectroscopy makes possible the discrimination between two adsorption modes, depending on whether or not the adsorption leads to breaking the Si-Si dimers. The observation of two different optical features is assigned to the bonding on dangling bonds or to the breaking of dimers, and gives access to the adsorption mode of hydrogen, water, oxygen, and pyridine. Moreover, the technique being quantitative, we can determine the total amount of dimers involved in the adsorption and monitor the adsorption kinetics.