The surface composition and structure of model Ziegler-Natta catalysts, polymerizing α-olefins to produce polyolefins, have been studied using modern surface science techniques and compared with their polymerization behaviors. Two types of thin films - TiClx/MgCl2 and TiCly/Au - were fabricated on an inert gold substrate, using chemical vapor deposition methods, to model the high-yield catalysts of MgCl2-supported TiCl4 and TiCl3-based catalysts, respectively. The model catalysts could be activated by exposure to triethylaluminum (AlEt3) vapor. Once activated, both catalysts were active for polymerization of ethylene and propylene in the absence of excess AlEt3 during polymerization. The model catalysts had polymerization activities comparable to the high-surface-area industrial catalysts. Though both catalysts were terminated with chlorine at the surface, each catalyst assumed different surface structures. The TiClx/MgCl2 film surface was composed of two structures: the (001) basal plane of these halide crystallites and a non-basal plane structure. The TiCly/Au film surface assumed only the non-basal plane structure. These structural differences resulted in different tacticity of the polypropylene produced with these catalysts. The TiClx/MgCl2 catalyst produced both atactic and isotactic polypropylene, while the TiCly/Au catalyst without the MgCl2 support produced exclusively isotactic polypropylene. The titanium oxidation state distribution did not have a critical role in determining the tacticity of the polypropylene.