Disulfur oxide (denoted as SSO) isolated in solid Ar at 13 K was irradiated with light at 308 nm from a XeCl excimer laser. New lines at 799.1 (797.0), 574.9, and 544.1 (545.6) cm−1, observed after photolysis, are assigned to cyclic S2O (denoted Cyc-S2O) with ∠S–O–S≅72.5±3.0° based on results of S34- and O18-isotopic experiments; lines in parentheses are associated with a minor matrix site and the broad line at 574.9 cm−1 may be deconvoluted to two lines at 575.4 and 574.6 cm−1. Secondary photolysis at 248 nm diminishes lines of cyc-S2O and produces SSO. Theoretical calculations using second-order Møller–Plesset theory with frozen core gradients and density-functional theories (Becke’s exchange functional with a correlation functional of Lee, Yang, and Parr) predict three stable isomers of S2O: cyc-S2O, SSO, and SOS, with the latter two having angular geometry. Relative energies, structures, vibrational wave numbers, and IR intensities were predicted for each isomer. According to calculations with Becke’s three-parameter exchange functional and the valence triplet-ζ basis set, cyc-S2O is bent with ∠S–O–S≅73.3° and has the S–S bond (2.058 Å) and both S–O bonds (1.724 Å) elongated relative to those of SSO (1.909 and 1.474 Å, respectively); it lies 41.3 kcal mol−1 above SSO. Isomer SOS, 62.0 kcal mol−1 greater in energy than SSO, has a S–O bond length 1.625 Å and ∠S–O–S≅128.5°. Calculated vibrational wave numbers, IR intensities, and isotopic shifts for cyc-S2O fit satisfactorily with experimental results. Two asymmetric transition states connecting SSO with SOS and cyc-S2O are characterized, yielding barriers for isomerization ∼104 and 122 kcal mol−1 (zero-point energy corrected), respectively. Photoconversion between angular SSO and cyc-S2O in a matrix cage is discussed. cyc-S2O might be responsible for some distinct features in thermal emission from the surface of Io, Jupiter’s moon.