InS features tetrahedral In(III) centers linked to four sulfido ligands.
α-InS has a defect cubic structure.  The polymorph undergoes a phase transition at 420 °C and converts to the spinel structure of β-InS.  Another phase transition at 740 °C produces the layered γ-InS polymorph.
β-InS has a defect spinel structure.  The sulfide anions are closely packed in layers, with octahedrally-coordinated In(III) cations present within the layers, and tetrahedrally-coordinated In(III) cations between them.  A portion of the tetrahedral interstices are vacant, which leads to the defects in the spinel.
β-InS has two subtypes.  In the T-InS subtype, the tetragonally-coordinated vacancies are in an ordered arrangement, whereas the vacancies in C-InS are disordered.  The disordered subtype of β-InS shows activity for photocatalytic H production with a noble metal cocatalyst, but the ordered subtype does not.
β-InS is an N-type semiconductor with an optical band gap of 2.1 eV. It has been proposed to replace the hazardous cadmium sulfide, CdS, as a buffer layer in solar cells, and as an additional semiconductor to increase the performance of TiO-based photovoltaics.
The unstable γ-InS polymorph has a layered structure.