Model Ziegler-Natta catalysts are prepared by gas phase deposition of ultra-thin TiCl4/MgCl2 films in UHV conditions. A monolayer of TiCl4 chemisorbed on a solid solution of titanium and magnesium chloride is formed in this way. The reduction and alkylation of TiCl4 by its reaction with a liquid layer of AlEt(3) condensed on the halide film is monitored by XPS. Most of the TiCl4 is reduced by AlEt(3) and is incorporated in the mixed titanium/magnesium chloride. The model catalyst is active in the polymerization of ethylene and propylene at 300 K, both in the absence and in the presence of AlEt(3) in the reaction cell. The polymers that form over the catalyst film have been characterized by Raman spectroscopy. The weak signals from methyl end groups and unsaturations suggest high molecular weight for both polymers. The polypropylene film has a high degree of isotacticity even without the use of any electron donor. For the propylene polymerization reaction the overall turnover frequency is in the range between 0.1 and 1 molecule/(site s).