A new low temperature modification of In 3 Ir, designated In 3 Ir-LT, was found which is stable below ~350 o C. Its crystal structure was determined using a single crystal κCCD-diffractometer at room temperature. Evaluations and refinements finally yielded a orthorhombic structure (space group: Pnma; Pearson symbol: oP16, Z=4) with a=7.456(1)A, b=8.314(1)A and c=5.313(1)A. For the entire data set of 1090 independent reflections residual values are R=0.0266 and R w =0.0572. The structure was found to be isotypic to Al 3 Ni with In on the Al-sites and Ir on the Ni-sites. Single crystals for structure determinations were gained by annealing of a sample with the composition In 8 0 Ir 2 0 at 320 o C (90 days), which contained In 3 Ir-LT and In after quenching. Several attempts to get pure bulk material of the low temperature phase by annealing samples prepared from the melt at 320 o C failed. The high temperature phase did not disappear completely even after repeated powdering and long time annealing. As there is no direct supergroup/subgroup relation between the low and high temperature modification (P4 2 /mnm, tPl6) of In 3 Ir a first order transformation mechanism was proposed.