Optimum conditions for the removal of phosphatic content in monazite sand by sintering with sodium carbonate are described in the previous papers in the series.† Results of investigation by D.T.A. and X-ray diffraction of the solid-solid reaction between monazite sand and sodium carbonate are now given. The D.T.A. curve shows that it is a slow exothermic reaction. X-ray diffraction patterns, besides confirming the constituents in the sintered product identified chemically, show the presence of a new phase.