[6] The wide band gap makes it transparent to infrared light between 620 nm and 11 μm.

The reaction can be represented with the chemical equation: As2S3 forms when aqueous solutions containing As(III) are treated with H2S.

When a film of this material is exposed to an external energy source such as thermal energy (via thermal annealing [9]), electromagnetic radiation (i.e. UV lamps, lasers,[10] electron beams)[11]), As4S6 polymerizes: As2S3 characteristically dissolves upon treatment with aqueous solutions containing sulfide ions.

[clarification needed] The dissolved arsenic species is the pyramidal trithioarsenite anion AsS3−3: As2S3 is the anhydride of the hypothetical trithioarsenous acid, As(SH)3.

[12] "Roasting" As2S3 in air gives volatile, toxic derivatives, this conversion being one of the hazards associated with the refining of heavy metal ores: Due to its high refractive index of 2.45 and its large Knoop hardness compared to organic photoresists, As2S3 has been investigated for the fabrication of photonic crystals with a full-photonic band-gap.

Arsenic trisulfide manufactured into amorphous form is used as a chalcogenide glass for infrared optics.

Arsenic trisulfide was used for the distinctive eight-sided conical nose over the infra-red seeker of the de Havilland Firestreak missile.

Precipitation of arsenic trisulfide is used as an analytical test for presence of dissimilatory arsenic-reducing bacteria (DARB).

Aged samples can contain substantial amounts of arsenic oxides, which are soluble and therefore highly toxic.