In order to provide a quantitative basis to discuss the efficiency of purification of organic mixtures achieved by layer of suspension type melt crystallization operations, experiments of purification by sweating were carried out with naphthalene-benzothiophene solid solution mixtures. The rate and the degree of composition changes were analyzed for crude crystals prepared either as a layer on a cold surface or as a suspension in a supercooled melt. The purification by sweating was conducted at constant temperatures (isothermal sweating) and the rate of composition change was evaluated by the purification rate coefficient k(p), which was defined as the ratio of the purification rate to the excess amount of the impurity in the crude crystal phase. The values of the purification rate coefficient for the layer type were found to be always levier than those for the suspension type, while the amount of impurities unable to be removed by sweating from the layer was larger than that for the suspension crystals. Consequently, it is concluded that purification by isothermal sweating in terms of the rate and the attainable purity was higher for the suspension type than the layer one in the present experiments. These results and the significance of the purification rate coefficient were discussed quantitatively.