The thulium nickel indide TmNiIn forms solid solutions TmNi 1−x−y In 1+x . Several samples have been prepared by arc-melting of the elements under argon. The structure of TmNiIn contains two crystallographically different nickel sites. The Ni1 atoms have a trigonal prismatic coordination by indium, while the Ni2 sites have six thulium neighbors in a trigonal prismatic arrangement. The Ni1 sites show defects in the solid solution, while the Ni2 sites have Ni2/In mixing with a maximal occupancy of 32 at.-% indium. The structures of three single crystals of solid solutions have been refined, leading to the compositions TmNi 0.88 In 1.10 (a =747.06(7), c = 367.8(1) pm, wR2 =0.0342, 323 F 2 values, 16 variables), TmNi 0.80 In 1.16 (a= 752.94(7), c =366.5(1) pm, wR2=0.0475, 503 F 2 values, 16 variables), and TmNi 0.76 In 1.21 (a = 758.4(1), c = 366.68(7) pm, wR2 = 0.0949, 226 F 2 values, 16 variables). The crystal chemical peculiarities and the differences in chemical bonding are briefly discussed.