Singly ionized sulfur monoxide (SO⁺) was detected by Turner in 1992 in the "shocked molecular clump" IC 443G. The observations were made with the NRAO 12m radio telescope. Turner suggested that SO⁺ was evidence for shock chemistry, that it was formed via a dissociative shock. However, van Dishoeck et al. countered that SO⁺ could be formed without a shock, if S⁺ is present in pre-shocked gas.

SO⁺ has also been detected toward the quasar PKS 1830-211, as reported by Muller et al in 2011. See Holger Müller's entry on extragalactic SO⁺ for more details about the detection.

When SO is ionized, the electron is removed from the sulfur 3p² orbital, leaving a phosphorus-like 3s² 3p_x 3p_y 3p_z configuration. That means that when O bonds to it, there is one σ bond and one π bond, with one singly occupied orbital leftover on sulfur. There are two ways to do this, as shown below in the bonding diagrams below. SO⁺ has a ²Π ground state, which gives it distictively doubled lines. The doubly and singly occupied valence π orbitals are also shown.