Polycrystalline U(Ni1-xCux)(2)Si-2 solid solutions have a tetragonal T
hCr2Si2 type crystal structure. Antiferromagnetic (AF) transitions, ob
served by measurements of a.c. susceptibility for x=0.10, 0.25, and 0.
50, are confirmed by neutron diffraction. The AF structures are of typ
e I (+-+-) with k=(0,0,1) below T-0=108+/-5 K for x=0.10 (with a possi
ble incommensurate phase at 108-135 K), T-N=151+/-3 K for x=0.25, and
T-N=150+/-2 K for x=0.50. The a.c. susceptibility transition for x=0.7
5 is associated with the three magnetic structures observed by neutron
diffraction: AF I, ferrimagnetic (++-) and AF IA (++--), with k=(0,0,
1), (0,0,2/3) and (0,0,1/2), and T-N=120+/-3, 115+/-4 and 110+/-3 K re
spectively. Including previous neutron results on single-crystal UNi2S
i2 and powdered UCu2Si2, a magnetic phase diagram of the U(Ni1-xCux)(2
)Si-2 system is proposed and discussed.