Supersymmetric Yang-Mills theory at order α'3

We construct the order α'3 terms in the supersymmetric Yang-Mills action in ten dimensions for an arbitrary gauge group. The result can be expressed in terms of the structure constants of the Yang-Mills group, and is therefore independent of abelian factors. The α'3 invariant obtained here is independent of the α'2 invariant, and we argue that additional superinvariants will occur at all odd orders of α'.