The catalytic properties of rhodium complexes containing the α‐, β‐, or γ‐amino‐phosphane ligands Ph2PCH2NEt2 (α‐P,N‐1), Ph2PCH(Ar)NHPh [α‐P,N‐2; Ar = η6(o‐C6H4Cl)Cr(CO)3], Ph2PCH2NPh2 (α‐P,N‐3), Ph2PCH2CH(Ph)NHPh (β‐P,N), Ph2PCH2(o‐C6H4–NMe2) (γ‐P,N‐1), Ph2PCH(o‐C6H4–CH2NHPh) (γ‐P,N‐2), and the α,β‐diamino‐phosphane ligand Et2NCH2P(Ph)CH2CH(Ph)NHPh (α,β‐N,P,N), in styrene hydroformylation have been examined. The results show that the activity increases when the number of backbone carbon atoms linking P and N decreases from 3 to 1. IR and 31P HPNMR studies in solution show that all P,N ligands adopt exclusively a κ1‐P coordination mode in rhodium chloride carbonyl complexes under high CO pressure. In the solid state a κ1‐P‐α‐amino‐phosphane coordination has been ascertained by X‐ray methods in trans‐[RhCl(CO)(γ‐P,N‐1)2]. In contrast, an equilibrium between the κ2‐P,N and κ1‐P‐coordination modes has been observed as a function of the CO pressure for the complex containing the β‐P,N ligand. The basicity of the dangling amino group also plays an important role on the catalytic activity and a mechanism involving the nitrogen function in the catalytic cycle is proposed.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)