Depending on the. manufacturer, some 1 kg bags of commercial plaster of Paris have little holes in order to decrease their volume for storage. Because this exposes the plaster to the humidity of the air, there is a partial conversion of the plaster to calcium sulfate dihydrate, changing the original quality of the product. This work has, as its objective, the development of methods to estimate the contents of calcium sulfate hemihydrate, calcium sulfate dihydrate and inert components of the plaster by use of dynamic or isothermal thermogravimetry (TG). Using dynamic TG the different contents can be measured directly by one analysis, otherwise, they can be measured by the combined results of the two isothermal analyses, of the original sample, and of a modified sample obtained after total hydration and a drying step of the original one. In isothermal analysis dehydrations of both salts occur simultaneously and there are insufficient data to calculate the contents by only one analysis. The methods and the results obtained for the two types of plasters are shown and discussed, indicating that the analyzed samples have from 82 to 89 mass % of hemihydrate, a content from 5 to 10 mass % of the dihydrate and from 5 to 7 mass % of inerts.