1. cis-4-Decenoyl-CoA, an intermediate of linoleic acid catabolism, is degraded by a soluble enzyme fraction of beef liver mitochondria to octanoyl-CoA. cis-2-Octanoly-CoA is not observed among the intermediates of this degradation sequence. 2. The existence of a mitochondrial 4-enoyl-CoA reductase which is distinct from the 2-enoyl-CoA reductase is demonstrated in beef liver. 3. Substrates for the 4-enoyl-CoA reductase are acyl-CoA esters, which possess a 2,4-diene structure rather than those containing an isolated double bond in position 4. 4. The 4-enoyl-CoA reductase is involved in the catabolism of cis-4-decenoyl-CoA. 5. A reaction sequence for the degradation of cis-4-decenoyl-CoA to octanoyl-CoA is proposed which combines the 4-enoyl-CoA reductase with the 'classical' beta-oxidation enzymes.