Indium monochloride occurs as a yellow cubic form below 120 °C and above this temperature as a red orthorhombic form.

[3][4] According to X-ray crystallography, the structure of the yellow polymorph resembles that of sodium chloride except that the Cl-In-Cl angles are not 90°, but range between 71 and 130°.

The red (high T) polymorph crystallizes in the thallium(I) iodide motif.

[5][6] The relatively high energy level of the 5s electrons of the indium center make InCl susceptible to oxidation as well as disproportionation into In(0) and InCl3.

[2] Tetrahydrofuran (THF) appears to facilitate the disproptionation of InCl as well as other indium(I) halides.