Abstract A sample of the coal is partially demineralized by treatment with hydrochloric and hydrofluoric acids under such conditions that coal substance remains unaffected. The loss in mass of the coal due the acid treatment is recorded and the undissolved part of the mineral matter is determined by ashing the partiallay demineralized coal. The iron copntent of the ash is determined so that the pyrites present in the extracted coal can be calculated. In addition the amount of hydrochloric acid absorbed by the coal substance is determined.