The point of zero charge (PZC) for monazite and xenotime reported in the literature has a wide range. Chemical modelling from this study shows that variation in the chemical composition of the monazite and xenotime samples can have detrimental effects on the chemical speciation of the mineral surfaces. It has been found that any slight variation of the chemical composition of the mineral surface would therefore, cause a reversal of surface charge. This explains the wide variation of PZC of different rare earth mineral samples from different mines.