A sensitive method is presented for the investigation of planar Ziegler-Natta model catalysts by means of attenuated total internal reflection Fourier transform infrared spectroscopy. A solution of MgCl2 in ethanol, optionally containing an internal donor, was spin-coated directly onto a planar ZnSe ATR crystal. Ethylene and propylene polymerization was monitored in situ and ex-situ, respectively, and interaction of a diester internal donor with MgCl2 and TiCl4 was investigated. The isotacticity of the polypropylene produced in the presence and absence of donor was estimated based on characteristic peaks in the infrared spectra.