By reaction of solid As4S4 with gaseous Cl2 at a temperature of 410 K gaseous AsSCl and AsS2Cl are formed. Unexpectedly in AsS2Cl the arsenic is not of formal oxidation state +V but +III: the molecular structure of AsS2Cl is arranged as a 1-chloro-dithia-arsirane and comprises an hitherto unknown AsS2 three-membered ring. Thermodynamic data on AsSCl and AsS2Cl are obtained by mass spectrometry (MS). The experimental data are extended and confirmed by ab initio quantum chemical calculations (QC). The following values are given: ΔfH0298(AsSCl) = −5.2 kJ mol−1 (MS), ΔfH0298(AsSCl) = 1.7 kJ mol−1 (QC), S0298(AsSCl) = 296.9 J K−1 mol−1 (QC) and cp0T(AsSCl) = 55.77 + 3.97 × 10−3T − 4.38 × 105T−2 − 1.83 × 10−6T2 and ΔfH0298(AsS2Cl) = −39.0 kJ mol−1 (MS), ΔfH0298(AsS2Cl) = −20.2 kJ mol−1 (QC), S0298(AsS2Cl) = 321.3 J K−1 mol−1 (QC) and cp0T(AsS2Cl) = 80.05 + 5.09 × 10−3T − 7.61 × 105T−2 − 2.35 × 10−6T2 (298.15 K < T < 1000 K) (QC). The ionization energies are determined (IP(AsSCl) = 10.5, IP(AsS2Cl) = 10.2 eV). The IR spectrum of AsSCl is detected by means of matrix isolation spectroscopy. The estimated force constant f(As[double bond, length as m-dash]S) = 4.47 mdyn·Å−1 gives rise to an As[double bond, length as m-dash]S double bond.