This is a report on31P nmr spectra of phosphoric acid monodecylester, PDE, and2H quadrupolar splitting of D2O in a lyomesophase developed from n-decylammonium chloride, PDE/H3PO4and D2O.Polarizing microcopic and2H nmr studies showed that the mesophase was of lamellar-like structure but alignes as the temperature is increased. As the mesophase contained a minute amount of free H3PO4, the31P nmr spectra gave a sharp peak and a hump. The latter was interpreted to arise from a slow tumbling process of PDE between different agglomerates in this system.