The use of silicone oil as an oxygen vector for increasing citric acid production by free and immobilized Aspergillus niger conidiospores was studied. When silicone oil was used, citric acid concentration increased with respect to the control run in free and immobilized systems 2.0 and 1.6 times, respectively. The effect of potassium ferrocyanide [K4Fe(CN)(6)] on citric acid production in a medium containing air, oxygen and silicone oil was studied. When K4Fe(CN)(6) was used with oxygen and silicone oil, citric acid concentration decreased significantly because of the conversion of ferrocyanide to ferricyanide. The reuse of immobilized A. niger conidiospores was investigated in citric acid production medium containing 2% (v/v) silicone oil. Citric acid production decreased on increasing the reuse number. (C) 2002 Elsevier Science Ltd. All rights reserved.