Intermediate states of degradation of phosphorus pentoxide in dimethyl sulfoxide (DMSO), also known as Onodera reagent, are studied. We found that DMSO is not dissolving P4O10, but rather reacting with it. A rather complex mixture of phosphate species is formed, many with ester functions. Several not yet described phosphate species could be identified by NMR and MS. Finally, we present a possible decomposition scheme of P4O10 in DMSO.