The problem of branch line strengths in 1 Δg—3 Σelectric quadrupole (E2) transition is addressed in generality. This is an intercombination transition, which derives its strength from spin–orbit perturbations. Considerations based on this premise lead to line strength expressions involving two transition moments Q21 and Q22. The formulas are used to analyze the intensities of the S- and O-branch transitions in the 1 Δg—X3 Σ-g, 0–0 band of the infrared atmospheric oxygen system measured recently (GordonI E, Kassi S, Campargue A,Toon GC. JQuant Spectrosc Radiat Transfer 2010;111:1174–83) [14]. The results show that although the major part of the E2 intensity component in the a_X, described by the moment Q22, is derived from the b1Σ+g - a1Δg Noxon transition, the smaller moment Q21 arising from remote perturbers also contributes to the intensity upto a few percent.