The synthesis of short chain S-methyl thioesters was investigated in Geotrichum candidum strain GcG. The results indicated the involvement of an enzymatic reaction in this microorganism that led to the synthesis of S-methyl thioacetate (MTA) when methanethiol and acetyl-CoA were used as substrates. MTA was generated from these substrates by enzymatic or spontaneous reactions, whose relative importance depended largely on pH and temperature. For longer chain acyl-CoA compounds (C3 to C6), thioester synthesis was primarily spontaneous. Short chain fatty acid activation by a CoA residue probably is a prerequisite for the synthesis of S-methyl thioesters. ß