Hydroformylation of methyl linoleate and linolenate with a rhodium-triphenylphosphine catalyst yields 1,4- diformyl esters as major products and 1,3-diformyl esters as minor products. Unsaturated and saturated monoformyl esters are also produced from both substrates and triformyl esters from methyl linolenate. Conjugated methyl linoleate produces mainly monoformyl esters, whereas methyl cis-9,cis-15-octadecadienoate produces diformyl esters. The mechanism postulated for dihydroformylation involves cyclic unsaturated acyl-rhodium-triphenylphosphine complexes controlling the position of the second formyl group. These intermediates are also implicated in double-bond hydrogenation and isomerization which accompany dihydroformylation. The polyformyl ester products formed contrast with the monoformyl ester products obtained when cobalt carbonyl catalysts are used.