Adsorption onto the surface of silica gel is studied by using the ab initio molecular orbital (MO) method. We have carried out MP2/6-31G(p,d)//HF/6-31G(p,d) level calculations to investigate gas-phase adsorptive interactions between adsorbates and silica gel. For adsorbates we have treated C2H4 and C6H6, which have -C=C- bonds, and for comparison NH3 has also been treated. In order to discuss preferable adsorption sites for these adsorbates, we treat three types of sites, which are constructed by an isolated =Si-O-H group, two =Si-O-H groups, and a =Si-O-Si= group. It is found that for C2H4, C6H6, and NH3 the former two types of sites, which are constructed by =Si-O-H groups, act as adsorption sites, but the site constructed by =Si-O-Si= group does not. A preferable adsorption site depends on each adsorbate. For C6H6 the paired-type adsorption site, which is constructed by two -OH groups, is preferable. In the adsorption of NH3, the single-type adsorption site, which is constructed by one -OH group, is preferable. We can deduce that the distribution of -OH groups strongly affects the adsorption phenomena and the character of the adsorbent.