Essentially complete regioselectivity and good catalytic activity can be achieved in the copolymerization of propene with carbon monoxide using palladium-containing catalytic systems modified by the basic diphosphine ligand CH2{CH2P(C2H5)2}2. With (6,6’-dimethylbiphenyl-2,2-diyl)bis(dicyclohexylphosphine) a stereoregular poly(1-oxo-2-methyltrimethylene) was obtained. When the optically pure ligand was used, the produced copolymer showed an intense band in the circular dichroism spectrum in the region around 275 nm as expected for a prevailingly isotactic structure. In most cases the copolymers can be isolated as a poly[spiro-2,5-(3-methyltetrahydrofuran)]. NMR analysis of the regioregular copolymers and comparison with model compounds suggest that the insertion of the propene units during the growth process takes place with primary regioselectivity.