The DSC-DTA combination technique has been used to estimate CaSO4. 2H2O, Mg(OH)2, Ca(OH)2, CaCO3 and MgO, comprising three white coat plasters. Unhydrated magnesium oxide could be estimated by converting it to Mg(OH)2 by autoclaving. An equation is derived from which unhydrated MgO can be determined by substituting the estimated values for CaSO4. 2H2O, Mg(OH)2, Ca(OH)2 and CaCO3. Estimated values of CaO, MgO and CaSO4. 2H2O obtained by the DSC-DTA technique correspond closely to those determined by chemical analysis.