This paper describes the synthesis of 2-ethylheptanoic acid (2) by hydrogenation of the delta-lactone (1), which is on his part very well available by the reaction of butadiene with carbon dioxide. A two-step hydrogenation process was found to be the most successful reaction pathway. First of all the cleavage of the delta-lactone ring is applied. This process involves a homogeneously catalyzed two-phase hydrogenation with a water-soluble in situ rhodium-phosphine catalyst. The effects of ligand, temperature, hydrogen pressure and catalyst concentration have been examined. The catalyst containing aqueous phase has been recycled five times without loss of activity. The leaching of the rhodium into the organic phase was measured and found to be in the range of 1 wt.% of the starting catalyst concentration. The hydrogenation of the remaining double bonds was carried out with a heterogeneous palladium/charcoal catalyst in methanol. (C) 2002 Elsevier Science B.V. All rights reserved.