The hydroformylation of vegetable oils (soybean, high oleic safflower, safflower, and linseed) and model compounds (methyl linoleate, methyl oleate, triolein, and trilinolein) using Rh(CO)(2)Acac ((acetylacetonato)dicarbonylrhodium(l)) as the catalyst precursor in the presence of triphenylphosphine (TPP) or (PhO)(3)P converted the olefins to polyaldehydes in quantitative yield. The reaction rate expressed as either initial rate or maximum turnover frequency (TOF) correlates with the olefin concentration of the substrate independent of the type of dominant triglyceride in vegetable oils. The relative rate constants (k(0)/k(1)) between methyl oleate and methyl linoleate at 90 degreesC and 1000 psig measured separately (2.4) and measured competitively in a mixture (0.90 +/- 0.08) indicate the formation of a pi-allyl complex of the catalyst with the diolefin, methyl linoleate. The relative rate constants (k(0)/k(1)) of triglycerides (triolein and trilinolein) measured separately (1) and measured competitively in a mixture (0.94 +/- 0.06) do not indicate any significant effect from pi-allyll complex formation on the reaction speeds of vegetable oils. The co-catalyst, (PhO)(3)P, showed a lesser reactivity compared to TPP in contrast to the rates of bulky phosphite ligands reported in the literature. (C) 2002 Elsevier Science B.V. All rights reserved.