Temperature-programmed desorption (TPD) of physisorbed mesitylene molecules has been used as a nondestructive surface probe to distinguish the surface adsorption sites of model Ziegler-Natta polymerization catalysts. A MgCl2-supported titanium chloride film (TiClx/MgCl2) was fabricated on an inert gold substrate by codeposition of Mg metal and TiCl4 from the gas phase. The mesitylene TPD probe revealed two types of surface adsorption sites. The dominant site was attributed to the basal plane of these halide crystallites. The minor site could be tentatively attributed to a defective structure at the basal plant boundaries or other crystal planes. Due to the chlorine termination nature of the catalyst surface, the metal ions under the chlorine layer could not be differentiated directly with the physisorbed mesitylene molecules. However, the mesitylene TPD profile was able to monitor compositional changes in the outermost chlorine layer accompanying the reaction of the catalyst film with the triethylaluminum cocatalyst, electron beam irradiation of the film surface, and diffusion of bulk chlorine to minimize the number of defect sites.