The copolymerization of 1-hexene and propylene in the presence of a Ziegler-Natta type catalyst (AlEt2Cl-TiCl3) has been studied. The gas chromatography method used for the kinetic study allowed determination of reactivity ratios of both monomers from the classical copolymerization equation which was found to be followed. The copolymers composition has been confirmed by NMR analyses and the theoretical sequence distribution computerized, thus giving an a priori description of the macromolecules useful for further studies of the properties of these copolymers. The drift in copolymer composition and thus in the distribution of monomer units occurring with conversion has been found important, and to avoid this heterogeneity in composition, some copolymers were prepared at constant monomer feed composition using a very simple manometric device.