Low molecular weight homopolypropylene and ethylene-propylene copolymers with low ethylene content have been produced with a highly stereoselective metallocene catalyst in the presence of dihydrogen. Quantitative C-13 NMR analysis allowed the detection of regioirregular (2,1-insertions) propylene units both in propylene homosequences and in ethylene-propylene sequences, and of different types of end groups. The microstructure of the polymers has been correlated with polymerization mechanisms, and the effect of ethylene on polymer regioregularity, termination and initiation steps has been investigated. The amount of 2,1-misinserted propylene units followed by one ethylene unit increases with increasing ethylene content in the polymer, which confirms that ethylene insertion after a 2,1-misinsertion is favored over propylene 1,2-insertion. Additionally, addition of ethylene reduces the frequency of secondary insertions. Under the investigated polymerization conditions, the dominating termination route was chain transfer to H-2 after 1,2- or 2,1-inserted propylene unit. In the presence of ethylene, the relatively fast ethylene insertion decreased significantly the probability of chain termination after 2,1-inserted propylene unit. Chain initiation took place by propylene 1,2-or 2,1-insertion into the Zr-H bond, and no signs of initiation by ethylene unit were detected. The initiation step was partly regioselective and favored 1,2-propylene insertion over 2,1-insertion.