The deuterioformylation of 1-hexene catalyzed by (chelating diphosphine)rhodium catalysts gave nearly exclusive formation of CH3(CH2)(3)CHDCH2CDO and CH3(CH2)(3)CH(CH2D)CDO, with the deuterium label at the beta-carbon and at the aldehyde carbon. Very little deuterium was incorporated into recovered hexenes. These results establish that the regiochemistry of aldehyde formation is set by a largely irreversible addition of a rhodium hydride to complexed 1-hexene to produce an alkylrhodium intermediate that is committed to aldehyde formation. Plausible steric explanations for the increased n:i aldehyde regioselectivity seen for diequatorial chelates compared with apical-equatorial chelates are considered. However, such steric explanations are not supported by molecular mechanics calculations of probable transition states.