The metabolism of isoleucine to active amyl alcohol (2-methylbutanol) in yeast was examined by the use of 13 C nuclear magnetic resonance spectroscopy, combined gas chromatography-mass spectrometry, and a variety of mutants. From the identified metabolites a number of routes between isoleucine and active amyl alcohol seemed possible. All involved the initial decarboxylation of isoleucine to ␣-keto-␤-methylvalerate. The first, via branched chain ␣-ketoacid dehydrogenase to ␣-methylbutyryl-CoA, was eliminated because abolition of branched-chain ␣-ketoacid dehydrogenase in an lpd1 disruption mutant did not prevent the formation of active amyl alcohol. However, the lpd1 mutant still produced large amounts of ␣-methylbutyrate which initially seemed contradictory because it had been assumed that ␣-methylbutyrate was derived from ␣-methylbutyryl-CoA via acyl-CoA hydrolase. Subsequently it was observed that ␣-methylbutyrate arises from the non-enzymic oxidation of ␣-methylbutyraldehyde (the immediate decarboxylation product of ␣-keto-␤-methylvalerate). Mutant studies showed that one of the decarboxylases encoded by PDC1, PDC5, PDC6, YDL080c, or YDR380w must be present to allow yeast to utilize ␣-keto-␤-methylvalerate. Apparently, any one of this family of decarboxylases is sufficient to allow the catabolism of isoleucine to active amyl alcohol. This is the first demonstration of a role for the gene product of YDR380w, and it also shows that the decarboxylation steps for each ␣-keto acid in the catabolic pathways of leucine, valine, and isoleucine are accomplished in subtly different ways. In leucine catabolism, the enzyme encoded by YDL080c is solely responsible for the decarboxylation of ␣-ketoisocaproate, whereas in valine catabolism any one of the isozymes of pyruvate decarboxylase will decarboxylate ␣-ketoisovalerate.