Two samples of Pliocene lignites from the Ptolemais basin of Greece, one from the upper and one from the lower lignite seams, were heated and dried in air at 50 degrees C intervals from 50 to 1200 degrees C. The two lignite samples initially contained the same minerals, namely calcite, dolomite, quartz, kaolinite, illite, pyrite and gypsum, but in different proportions. The lignite sample from the upper lignite seam is rich in Fe2O3, CaO and SO3, while that from the lower lignite seam is rich in SiO2 and Al2O3. Hematite, periclase, melilites, merwinite, calcium ferrite and brownmillerite are constituents of the 1200 degrees C lignite ash from both samples. The heating conditions and the chemistry of the samples lowered the formation temperatures of brownmillerite, which appeared in both samples at 950 degrees C. In the Fe2O3, CaO- and SO3-rich sample, magnesioferrite is present from 850 to 1100＇＇C and hematite appears at 300 degrees C. In the SiO2- and Al2O3-rich sample, magnesioferrite was not detected at any temperature and hematite appeared at 600 degrees C. Anhydrite, which normally decomposes in air at 1638 degrees C, is the main constituent at 1150 degrees C, on heating the lignite sample that was rich in Fe2O3, CaO and SO3. Anhydrite diminishes at 1200 degrees C. In the SiO2- and Al2O3-rich lignite sample, anhydrite is main constituent at 1100 degrees C, but diminishes considerably at 1150 degrees C and decomposes at 1200 degrees C.