Diamond has potential as a wide band-gap semiconductor with high intrinsic carrier mobility,thermal conductivity and hardness. Hydrogen is involved in electrically active defects inchemical vapour deposited diamond, and muonium, via muon spin spectroscopy, canprovide useful characterization for the configurations adopted by H atoms in a crystallinematerial. We present the results of a computational investigation into the structure of theMuX centre proposed to be associated with nitrogen aggregates. We find that the propensity ofhydrogen or muonium to chemically react with the lattice makes the correlation ofMuX with nitrogen aggregates problematic, and suggest alternative structures.