Abstract The complexation of Co(II) and Ni(II) by citrate in 0.3–5 m NaCl solutions has been investigated. The stability constants for CoCit − and NiCit − were measured as a function of NaCl concentration using a solvent extraction method with HDEHP in toluene. The stability constants for both complexes decreased with increasing NaCl concentrations up to 5 m. The data were modeled using the ion-interaction formalism developed by Pitzer. The dimensionless standard chemical potentials for the CoCit − and NiCit − complexes were calculated to be −2.561 and −0.697, respectively. Binary interaction parameters calculated in this work were β (0) =−0.2249 and C ϕ =0.0539 for NiCit − and Na + interactions and β (0) =−0.5407 and C ϕ =0.1354 for CoCit − and Na + interactions. Because of limited data, the β (1) value for both CoCit − and NiCit − with Na + was fixed at an average value of 0.29 which was obtained by averaging 1–1 interactions for other carboxylic acid salts. Higher order interaction parameters were not required to model the data.