Linear carbon disulfide (denoted as SCS) isolated in solid N2 or Ar at 13 K was irradiated with light at 193 nm from an ArF excimer laser. In addition to an absorption line of CS at 1277.4 cm-1, new lines at 881.3 and 520.9 cm-1 were observed after photolysis of SCS in solid N2. These lines are assigned to cyclic CS2 (denoted cyc-CS2) based on results from 34S- and 13C-isotopic experiments. Doublet lines of cyc-CS2 at 876.5 (881.1) and 517.7 (522.7) cm-1 were observed after irradiation of SCS in solid Ar at 193 nm; lines in parentheses are associated with a minor matrix site. Secondary photolysis at 248, 308, 532, 560, or 580 nm diminishes signals of cyc-CS2 and produces SCS. Theoretical calculations using MP2-full and density-functional methods (BLYP and B3LYP) predict three isomers of CS2: SCS, cyc-CS2, and linear CSS; relative energies, structures, vibrational wavenumbers, and IR intensities were predicted for each isomer. Cyc-CS2 has C-S bonds (~1.74 Å) elongated relative to those of SCS (~1.56 Å), a S-S bond ~2.14 Å, and angleSCS ≃ 76°; it lies ~73 kcal mol-1 above SCS. Calculated vibrational wavenumbers, IR intensities, and isotopic shifts for cyc-CS2 fit satisfactorily with experimental results. An asymmetric transition State Connecting SCS and cyc-CS2 is characterized, yielding a ring-opening barrier of ~24.4 kcal mol-1 (zero-point energy corrected). Photoconversion between linear and cyclic CS2 in a matrix cage is discussed.