The crystal structure and characterization in solution are described for a very regioselective hydroformylation catalyst, RhH(CO) 2 (diphosphite) with {2(3,3 ,5,5 -tetra-tert-butyl-1,1 -biphenyl-2,2 -diylphosphite),2 (bis( 4-Cl-phenyl)phosphite)-3,3,5 ,5 -tetra-tert -butyl-1-1 -biphenyl} as the diphosphite. The complex is formed under CO-H 2 pressure from Rh(CO) 2 Acac and has a distorted TBP geometry with both phosphorus atoms in equatorial positions.