Chelating diphosphines with large P-M-P bite angles near 120 degrees are interesting in the development of catalysts with controlled local geometry. In rhodium catalyzed hydroformylation, the key HRh(CO)₂L₂ catalysts are known to be mixtures of diequatorial diphosphine and equatorial-apical diphosphine complexes. We set out to test the hypothesis that very different regioselectivities of aldehyde formation might be obtained from diequatorial diphosphine rhodium complexes and from equatorial-apical diphosphine rhodium complexes. We found a strong correlation between regioselectivity for n-aldehyde formation and natural bite angle. Chelates like BISBI with wide natural bite angles gave a much higher % n-aldehyde than diphosphines with narrow bite angles. Attempts to explain this selectivity by steric effects alone have failed and the role of electronic effects of chelating ligands is being investigated.