The microstructures of polypropenes produced with several zirconocene-based catalyst systems are compared, to verify the possible correlation between the type of stereospecificity and the amount of regioirregularities. It is confirmed that, while syndiospecific and aspecific zirconocenes are highly regiospecific, isospecific systems produce substantial amounts of regioirregular monomeric units. The amount of these secondary units strongly depends on the nature of the pi-ligands and on the type of the bridge connecting them. Molecular mechanics calculations are reported, indicating that the intermediates which are energetically suitable for the secondary and primary insertions, for isospecific or syndiospecific complexes, coordinate monomer enantiofaces of the opposite or the same chirality, respectively. This difference accounts for the lower regiospecificity of the isospecific catalytic complexes, assuming that the energy barrier for the rotation of the coordinated monomer around the metal-olefin bond, between the orientations suitable for the primary and secondary insertions is lower than (or comparable to) the activation energy for secondary monomer insertion.