A series of hydroformylation experiments was performed with a high-grade and a technical-grade-derived methyl oleate (MO) and a rhodium catalyst modified by the bulky tris (2-tert-butyl-4-methylphenyl)phosphite. In the hydroformylation of pure methyl oleate, relatively high turnover numbers were obtained (400-500 mol/mol/h) under mild conditions (molar ratio MO/Rh = 910, 80-100°C and 20 bar; CO/H 2 = 1:1, solvent toluene), leading to about 95% conversion in 3 h. Fast isomerization occurs under these conditions to produce the trans oleate. Trans oleate reacts more slowly than cis oleate. At temperatures below 50°C, isomerization does not occur. The use of technical-grade methyl oleate, containing 14% 9,12 diene, methyl linoleate (ML), results in lower reaction rates because dienes form stable π-allylic intermediates, which slowly undergo hydroformylation. More severe conditions were applied to obtain higher rates. The rate varied from 50 to 400 mol/mol/h, depending on conditions (molar ratio MO/Rh = 910, T = 50-120°C, P = 50-80 bar; CO/H 2 = 1:1-1:6, solvent, toluene). Several isomers of ML were formed during the reaction. Subsequent hydroformylation of these isomers results in a complicated mixture of products. The product mixture consists predominantly of methyl formylstearate, methyl formyloleate, methyl diformylstearate, and some yet unidentified side products. A comparison of the classic triphenylphosphine-modified catalyst and the bulky phosphite-modified catalyst has shown that the latter is several times more active. JAOCS 74, 223-228 (1997).