BACKGROUND: Synergistic extraction has been proven to enhance extractability and selectivity. Numerous types of synergistic extraction systems have been applied to rare earth elements, among which sec-nonylphenoxyacetic acid (CA100) has proved to be an excellent synergistic extractant. In this study, the synergistic enhancement of the extraction of holmium(III) from nitrate medium by mixtures of CA100 (H 2 A 2 ) with 2,2 -bipyridyl (bipy, B) in n-heptane has been investigated. The extraction of all other lanthanides (except polonium) and yttrium by the mixtures in n-heptane has also been studied. RESULTS: Mixtures of CA100 and bipy have significant synergistic effects on all rare earth elements, for example holmium(III) is extracted as Ho(NO 3 ) 2 HA 2 B with the mixture instead of HoH 2 A 5 , which is extracted by CA100 alone. The thermodynamic functions, H o , G o , and S o have been calculated as 2.96 kJ mol -1 , -6.23 kJ mol -1 , and 31.34 J mol -1 K -1 , respectively. CONCLUSION: Methods of slope analysis and constant molar ratio have been successfully applied to study the synergistic extraction stoichiometries of holmium(III) by mixtures of CA100 and bipy. Mixtures of these extractants have also shown various synergistic effects with other rare earth elements, making it possible to separate them. Thus CA100 + bipy may be used to separate yttrium from other lanthanides at appropriate ratios of the extractants.