Investigations carried out deal with the effect of inorganic and organic admixtures in different concentrations on the properties of ß-hemihydrate plasters of mineral gypsum (selenite) and chemical gypsum (phosphogypsum). It has been observed that the addition of these admixtures plays a vital role in the increase of the compressive strength and also improves water resistance of the plaster. Addition of 1 % K2SO4 increases about 54 % and 66 % of compressive strength of selenite and phophogypsum plasters respectively, while water absorption decreases upto 30 %. The order of increase in compressive strength and water resistance observed is Al2(SO4)3