Synergistic solvent extraction (SX) utilises combinations of extractants and synergists to enhance the selectivity of the organic extraction system. It has previously been shown by work conducted at the AJ Parker CRC/CSIRO Minerals that the selectivity of Versatic 10 for nickel over calcium was substantially improved by adding the Avecia reagent Acorga CLX50 to the organic system as a synergist. However, the chemical stability of this reagent under conditions relevant to an operating circuit had not been established. This study aimed to assess the chemical stability of Acorga CLX50 in a Versatic 10/Acorga CLX50/Shellsol 2046 system under conditions used at the Bulong Nickel SX circuit. This was achieved by monitoring physical and compositional changes in the organic solution with time when tested under the following conditions: continuous batch mixing with synthetic strip solution over 12 weeks at 40 degreesC, continuous batch mixing with synthetic strip solution over 16 days at elevated temperature (60 degreesC), and continuous extraction-stripping operation using a mixer/settler with synthetic aqueous leach solution and spent nickel electrolyte from the Bulong plant over 16 days at 40 degreesC. Minor changes in physical properties associated with the addition of Acorga CLX50 to a mixture of Versatic 10/Shellsol 2046 were observed, namely a decrease in interfacial tension and an increase in primary and secondary phase disengagement times after both organic continuous and aqueous continuous mixing. Importantly, no adverse changes in the physical properties of any of the organic solutions from the three tested systems were noted as a function of extended mixing time and/or mixing temperature. Similarly, no loss of Acorga CLX50 or ongoing generation of degradation products associated with the presence of Acorga CLX50 was observed in any of the organic solutions from the three tested systems, by gas chromatography or high performance liquid chromatography. From these results it can be concluded that the Versatic 10/Acorga CLX50/Shellsol 2046 organic system is chemically stable under the conditions tested and the addition of Acorga CLX50 does not, directly or indirectly, lead to the generation of species that adversely affect phase separation. (C) 2002 Elsevier Science Ltd. All rights reserved.