The separation of lanthanides from actinides in the TALSPEAK liquid–liquid distribution process is accomplished using an aminopolycarboxylate complexing agent, for example diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid (DTPA, CAS Reg. No. 67-43-6), in a low pH buffered aqueous phase in contact with an organic phase containing an extractant such as di(2-ethylhexyl)phosphoric acid (HDEHP, CAS Reg. No. 298-07-7). Literature measurements show that the partitioning of lanthanides to the organic phase falls with rising pH whereas thermodynamic equilibrium models suggest that, at pH above approximately 3.5, the partitioning should increase. In this study, the partitioning of Eu3+ between an aqueous phase (with NaNO3 background electrolyte, malonate buffer, and DTPA complexing agent), and an organic phase (HDEHP in n-dodecane) is measured from pH 2 to 4.5 and for ionic strengths from 0.25 to 1.0 mol kg–1. The measurements include systems with reduced (by 10×) concentrations of buffer, DTPA, and Eu3+. A Pitzer activity coefficient model of the aqueous mixture is developed based upon available osmotic and activity coefficient data, and stoichiometric equilibrium constants in different 1:1 electrolyte media over a range of ionic strengths. This enables the DTPA and buffer speciation, and complexation of Eu3+ by both DTPA and malonate, to be calculated for different solution compositions and pH. The measured distribution coefficients are consistent with model predictions up to pH 3.5 and, below this pH, vary little with ionic strength. At higher pH, the distribution coefficients at different ionic strengths deviate both from the model and each other, consistent with other reactions occurring in the organic phase than the simple exchange of lanthanide and H+ embodied in the TALSPEAK phase transfer reaction.