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 angleS-O-Scongruent to72.5+/-3.0degrees based on results of S-34- and O-18-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 Moller-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-zeta basis set, cyc-S2O is bent with angleS-O-Scongruent to73.3degrees and has the S-S bond (2.058 Angstrom) and both S-O bonds (1.724 Angstrom) elongated relative to those of SSO (1.909 and 1.474 A, 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 Angstrom and angleS-O-Scongruent to128.5degrees. 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 similar to104 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.