Bi3O2S3 is a newly discovered superconductor with a critical temperature ~4.5 K and an upper critical field of 4.8 T. It is highly unusual among the new layered structures in containing neither Cu nor Fe, yet showing superconductivity in an undoped state, and having a bandstructure which is semimetallic with a very low carrier density. How this will influence the superfluid stiffness of the condensate is an intriguing question that muon-spin rotation is ideally suited to answer.