Synergistic extraction of hexavalent uranium was studied using acidic extractants HTTA /HPMBP with two dif-ferent bifunctional neutral donors, DHDECMP and CMPO,from HNO3 medium at various fixed temperatures. The equi-librium constants for the organic phase addition reaction (log Ks) were correlated with the basicity (Kh) of the neutral donors. The data reported earlier for monofuctional neutral donors (DPSO, TBP, TOPO) were used for the comparison.A linear correlation between log Ks and log Kh was observed for U(VI)/HTTA/S(S=neutral donor) system. However,in U(VI)/HPMBP/S system, the observed log Ks values for bifunctional neutral donors were much lower than those ex-pected from linear correlation. This was attributed to the different mechanisms operative in the synergistic extraction i.e. substitution in the former vs. addition in the latter. These conclusions are also supported by the thermodynamic data obtained in the present studies. Nevertheless, it is seen that bifunctional neutral donors act only as monofunctional with both HTTA and HPMBP.