The first rhodium complexes with the anionic 2-(bis(2-methoxyphenyl)phosphino)benzenesulfonate (L) ligand have been synthesized and characterized. Selected complexes have been used as catalyst precursors for the homogeneous hydroformylation of 1-hexene, which has been carried out in preparative autoclaves and studied under operando conditions by means of both high-pressure NMR (HPNMR) and high-pressure IR (HPIR) spectroscopy. Operando spectroscopic hydroformylation experiments have shown that the catalyst precursors bearing either 1,5-cyclooctadiene or triethylamine as ancillary ligand are rapidly converted into di-carbonyl complexes. Rhodium(I) mono-and di-carbonyl complexes have been shown to be resting states of the catalytic cycle, most likely in equilibrium with other species containing either the zwitterionic ligand HL coordinated in the kappa(1)-O bonding mode or exclusively carbonyl ligands. (c) 2008 Elsevier B.V. All rights reserved.