For the first time the direct conversion of arsenic monosulfide (As ₄ S ₄ ) in low-temperature non-equilibrium radio frequency (40 MHz) argon plasma discharge at low pressure (0.1 Torr) was studied in terms of the preparation of arsenic sulfide films of different chemical content. In this case, arsenic monosulfide was used as the only precursor. The degree of arsenic monosulfide conversion, and, respectively, the chemical content of the samples was varied through a change of plasma parameters—the electron density and the temperature in the plasma discharge. The optical emission spectra of plasma of the (Ar–As ₄ S ₄ ) mixture at different energy inputs were studied to clarify the routes of initiation of chemical interaction. The possible mechanism has been assumed. The expansion of the IR transparency window due to the change of structural properties of the materials in the plasma was investigated and discussed.