In the current study, the evolution of the composition of fly ash (FA), since the beginning of operation of the lignite-fired power station of Amynteon-Filotas [Steam Electric Station (SES) Amynteon-Filotas] in Greece, is presented. The respective evolution of the feedcoal-composition is also showed. This power station began to operate in 1987. Before that, it was predicted that lignite fly ash would turn from strongly calcareous to barely pozzolanic. Moreover, the variation of sulphur (%) (both total and volatile) in feedcoal had revealed that fly ash would not often meat the specifications to be used in the production process of concrete, due to the excessive percentage presence of SO3. These predictions were eventually confirmed in the 23 years of operation of the power station. During these years SO3(%) kept alternating, rendering -in some cases- fly ash suitable for use in cement industry applications. The periodical changes of the SO3 concentration in fly ash were attributed to the free (volatile) sulphur of feedcoal as well as to the excess of FeS2, which had also a serious effect on the daily power production process (severe erosion of the metallic surfaces of boiler). A number of solutions for tackling this phenomenon are suggested in this study: discard of specific FA grain fractions, intense monitoring of the material and its effective mixture-homogenization and selective mining of lignite.