For any industrial application of a homogeneously catalysed reaction a complete recycling of the-in general expensive-catalyst should be guaranteed. Also a highly efficient reaction without any mass transport problems is strongly desired. After examining the technically interesting rhodium-catalysed cooligomerization of diunsaturated fatty substances with ethylene, a solution of these requirements can be fulfilled by using some organic chlorides as promoting agents in combination with the new recycling concept of "temperature-dependent solvent systems." Because of their physical properties the products 2 + 3-internal branched fatty substances-are a very interesting class of new lubricants which are expected to be biodegradable. These branched fatty substances could be synthesised in high yields of up to 98% and the turn over frequency (TOF) could be increased from 2 up to 220 h(-1). (C) 2002 Elsevier Science B.V. All rights reserved.