[en] Adduct formation between U(TTA)₄ and several neutral donor (S) was investigated by utilizing the changes in the absorption spectra of U(IV) resulting from the addition of neutral donors to a solution of U(TTA)₄. All the donors used in the present work from 1:1 adducts with U(TTA)₄. From the spectral changes, the equilibrium constants βsub(AB) for the adduct formation reaction viz U(TTA)₄ S reversible U(TTA)₄.S were calculated for a few neutral donors. The log βsub(AB) values obtained in benzene medium, are :TOPO (6.23), TBPO (6.13), TPPO (4.72), DBBP(4.04) TBP(3.04), TIOTP(1.27) and MIBK(-0.10) and a value of 3.98 for TOPO in chloroform medium. The adduct formation was found to result in increasing the coordination number of U(IV) from 8 in U(TTA)₄ to 9 in the adducts it forms with the neutral donors. Similar absorption spectral studies with U(DBM)₄ revealed that it forms much weaker adducts than the corresponding ones with U(TTA)₄. The absorption spectra of the organic extracts of U(IV) from perchloric acid with mixtures of HTTA and a neutral donor were identical with the spectra of the adduct between U(TTA)₄ and the neutral donor. This suggested that the adduct species viz U(TTA)₄.S is involved in the synergistic extraction of U(IV). The extraction constants relating to the synergistic extraction of U(IV), from aqueous perchloric acid into benzene solutions of HTTA and some of the neutral donors, were derived. (author)