In this theoretical study on rhodium-catalysed hydroformylation we examine an unmodified hydrido-rhodium(I) carbonyl system a together with three variants modified by the model phosphane ligands PF3 (System b) PH3 (system c) and PMe3 (system d), which show increasing basicity on the Tolman parameter scale. The olefinic substrate for all systems is ethene. Based on the dissociative hydroformylation mechanism, static and dynamic quantum-mechanical approaches are made for preequilibria and the whole catalytic cycle. Agreement with experimental results was achieved with regard to the predominance of phosphane monocoordination in systems b-d, different sensitivity of unmodified and modified systems towards hydrogen pressure and the early location of the rate-determining step. Neither the catalytic cycle as a whole nor olefin insertion as an important selectivity-determining step gives a clear picture of activity differences among a-d. However, the crucial first catalytic step, association of ethene to the active species [HRhL3] (L=CO, PR3), may play the key role in the experimentally observed higher activity of a and systems with less basic phosphane ligands modelled by b.