The absorption spectrum of SO radicals produced by flash photolysis of a mixture of COS + O2 + Ar is investigated. A partial rotational analysis of the previously known bands of the B3Σ−–X3Σ− transition which lie in the region of 1900 to 2400 Å is presented, and the predissociations and perturbations of the B3Σ−state are discussed. A complex red-degraded band system near 2500 Å, previously observed in emission and attributed to SO2, is shown to be due to a 3Π–X3Σ− transition of the SO molecule. Effective rotational constants of the 3Π state are derived from the analysis of these bands photographed at high resolution. In order to obtain the vibrational numbering of the 3Π–X3Σ− bands, these were also photographed in emission from a microwave discharge through a mixture of S18O2 + S16O2. A general discussion of the currently known states of the SO molecule is given.