The highly selective liquid-liquid extraction of the pertechnetate anion TcO4−from a nitrate matrix by the lipophilic redox-recyclable extractant HEP⩲NO3−[HEP = l,1′,3,3′-tetrakis(2-methyl-2-hexyl)ferrocene] has been investigated in comparison with the well known anion-exchange reagent tetra-n-heptyl ammonium nitrate (heptyl)4N+NO3−. Under conditions of low metal-anion loading, the distribution ratio D(TcO4−) was found to be insensitive to the alkali metal cation present as M+NO3−(M+= Li+, Na+, K+) and the initial concentration of TcO4−in the aqueous phase for both extractants. Also, slope analysis of the distribution data revealed approximate linear relationships between D(TcO4−) and the concentration of extractant in the organic phase (slope = 1.0) and the concentration of NaNO3in the aqueous phase (slope = −1) plotted on a full logarithmic scale, suggesting that the extraction of TcO4−occurs by a simple anion-exchange mechanism. Graphical analysis of partial loading data confirmed that both HEP+NO3−and (heptyl)4N+NO3−are predominantly monomeric in chlorobenzene under conditions of low metalanion loading. However, at an organic-phase TcO4−concentration of approximately 0.003 M, a sharp decrease in D(TcO4−) was observed, suggesting the formation of large aggregated species. At lower concentrations of TcO4−equilibrium modeling of the extraction data with the program SXLSQI was generally consistent with the simplest model of anion exchange, namely, that involving only the monomeric organic-phase ion pairs R+NO3−and R+TcO4−[R+= HEP+or (heptyl)4N+], To model extraction at aqueous nitrate concentrations as high as 3 M, it was found necessary 10 include the Pitzer mixing parameter θ(NO3−,·TcO4−), whose value (0.0435 ± 0.(039) upon refinement was found to be the same for both extractants.The logKexchvalues for TcO4−/NO3−anion exchange were found to be 3.84 ± 0.01 for HEP+NO3+and 3.69 ± 0.01 for (heptyl)4N+NO3−indicating that the selectivity for TcO4−over NO3−is approximately the same for both extractants, This is consistent with the hypothesis, based on electrostatic principles, that the selectivity in the observed anion-exchange process is favored by the large (and comparable) sizes of the cationic extractants, However, our analysis suggests that a further increase in cation size would not significantly improve TcO4−/NO3−selectivity in chlorobenzene, though selectivity could be so increased in diluents of lower dielectric constant.