The 0–0, 1–1, 2–2, and 0–1 bands of the b1Σ+–X3Σ− transition of the SO molecule have been observed in the afterglow produced when COS + O2 is pumped rapidly through a microwave discharge. The two strongest bands, 0–0 and 1–1, which lie respectively at 9549.08 and 9626.13 Å, have been photographed at high resolution and have been analyzed. Using the known X3Σ− rotational constants, the vibrational and rotational constants of the 1Σ+ state (Tc = 10 509.97 cm−1) have been determined: ωc′ = 1067.66 cm−1, Bc′ = 0.70262 cm−1, and rc′ = 1.5005 Å. Rotational intensity distributions for 1Σ+–3Σ− transitions are discussed. The a1Δ state of SO is predicted to lie at T ~ 6350 cm−1.