The sticking process ofCl2on the Si(100) surfaces has been studied by employing a molecular-beam method. Initial sticking probabilities,S0,were measured as a function of the incident energy,Ei,and the surface temperature,Ts.TheS0versusEias well asS0versusTscurves were analyzed with a kinetic model, which includes both a direct sticking channel and a precursor-mediated sticking channel. The physisorption lifetimes were measured as a function ofTsto evaluate the depth of the physisorption well and the preexponential factor for the detrapping rate. The potential structure and the intermediate transition state relevant to theCl2sticking on the Si(100) surface were discussed.