Disulfur monoxide dispersed in solid Ar at 15 K was irradiated with light at 308 nm from an XeCl excimer laser to form cyclic S2O, denoted cyc-S2O; cyc-S2O was identified according to infrared absorption lines reported previously. Using a similar technique to produce cyc-S2O, we recorded both infrared and ultraviolet absorption spectra of the same matrix sample before and after photolysis at various stages. A progression recorded in a range 330-370 nm with intervals ∼580 cm -1 correlates well with variations in intensities of infrared absorption lines of cyc-S2O. On the basis of characteristics of this observed progression and calculations of vertical excitation energy and oscillator strength using the time-dependent density-functional theory, we tentatively assign this progression to an electronic transition 2 1B1 ← X1A1 of cyc-S 2O; the observed progression is associated with the S-S stretching (ν2) mode in the excited state. The 2 1B1 state lies ∼77.7 kcal mol-1 (27 166 cm-1) above the ground electronic state.