Abstract Recent work has raised interest in the use of a LIX®63/Versatic 10/tributylphosphate (TBP) synergistic system to separate nickel and cobalt from gangue metals. However, accelerated and undesirable degradation of LIX®63 oxime has previously been reported in the presence of strong organic acids such as di-2-ethylhexyl phosphoric acid (D2EHPA). Although not a strong organic acid, no reliable information regarding the effect of Versatic 10 on LIX®63 stability was available. The present work assessed the effect of organic composition on the stability of LIX®63 oxime when mixed for 6 weeks under harsh strip conditions (20 g/L sulfuric acid, 60 °C). The organic solutions were (i) LIX®63 in Shellsol D70; (ii) LIX®63 and D2EHPA in Shellsol D70; (iii) LIX®63 and Versatic 10 in Shellsol D70; and (iv) LIX®63, Versatic 10 and TBP in Shellsol D70. The adverse effect of organic acids on the rate of LIX®63 oxime degradation was found to increase in the order: No organic acid ≪ Versatic 10 ≪ D2EHPA. Oxime decomposition was determined to be a first order reaction. The major degradation product was isolated, characterised and identified as 5,8-diethyl-6,7-dodecanedione (diketone). Loss of oxime from the LIX®63-only system occurred predominantly by ongoing precipitation of a nickel–oxime complex rather than by oxime degradation. Diketone formation accounted for less than 1% of total oxime loss. Oxime quickly decomposed in the presence of D2EHPA, with a half life of 6.5 days. Diketone, which in turn slowly decomposed, accounted for essentially all oxime loss. In contrast, only gradual loss of oxime from the LIX®63/Versatic 10/TBP (half life 34 weeks) system was observed, with diketone accounting for 61% of total oxime loss. Similar results were observed in the LIX®63/Versatic 10 system, suggesting the presence of TBP does not materially affect oxime stability or the degradation mechanism. The presence of diketone in the LIX®63/Versatic 10/TBP system was found to have no adverse effect on metal selectivity, although other degradation products could potentially have a small, adverse effect. The degradation products did not have any observable effect on interfacial tension or phase disengagement time. Additional studies to better identify factors affecting oxime decomposition (such as aqueous phase composition and temperature) in these synergistic systems are underway.