Control of an in-series, multiple, e.g., two, reaction zone, hydroformylation process for producing normal (N) and iso (I) aldehydes at a N:I ratio, the process comprising contacting an olefinically unsaturated compound, e.g., propylene, with carbon monoxide, hydrogen and a catalyst comprising (A) a transition metal, e.g., rhodium, (B) an organobisphosphite, and (C) an organomonophosphite ligand, the contacting conducted in first and subsequent reaction zones and at hydroformylation conditions comprising an olefinically unsaturated compound partial pressure in each zone, the control exercised by decreasing the olefinically unsaturated compound partial pressure in the first reaction zone to decrease the N:I ratio or increasing the olefinically unsaturated compound partial pressure in the first reaction zone to increase the N:I ratio.