We review the results of bulk and neutron diffraction measurements on the UCu5_xAux system crystallizing in the cubic AuBe 5 -type structure. The observed properties may be understood in the light of competition between Kondo effect and RKKY exchange interactions. Effects of 5f-ligand hybridization between the U-atoms occupying the 4a-sites and the Au-or/and Cu-atoms placed at the 4c-and 16e-positions, respectively, are supposed to be different. We show that the prefential occupation of

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... by Au has a marked effect on the magnetic properties of the investigated system. PACS numbers: 71.28.+d Among the UT5_xMx alloys, where T is Ni, Cu and Pt and M is Cu, Ag, Au and Pd, the UCu5_ x Mx phases attract much attention. One of the reasons is the fact that UCu5 is a heavy-fermion antiferromagnet [1] [2] [3] [4] . Moreover, this compound forms a gap in the electronic density of states below 1.2 K [1]. It has been interpreted that heavy fermions form a quasi-indepedent subsystem, which is associated with the weak magnetism or with spin-density-wave behaviour [4] . It has been recognized that the change of chemical composition in the UCu5 _x Mx system strongly influences its low temperature properties. In the work of van ball et al. [5] on UCu 5 _x Nix , it was pointed out that replacing Cu by Ni leads to rapid disappearance of the antiferromagnetic ordering. A similar effect has been observed by us in the UCu 5 _x Pdx alloys [6] . On the other hand, Ott et al. [1] have shown that when one Cu-atom in UCu5 was replaced by the Ag-atom, then the linear coefficient of the specific heat, y, extrapolated to T = O K rised from 86 mJ/mole K2 to 310 mJ/mole K2 and the Neel temperature increased from 15 K to 18 K. Replacement of Cu by Au in the UCu 5 _ x Aux system obviously should result in an analogous effect, because both the Ag and Au atoms have similar electronic configurations and atomic sizes. As expected, we have observed an increase in the Neel temperature from 15 K for UCu 5 to 27 K for UCu4Au [7] . However, a