The present work introduces and evaluates a new approach for the classification system of coal fly ashes (FAs) based on their origin, phase-mineral and chemical composition, properties, and behaviour. The detailed data for 41 FAs produced from various feed coals at 37 coal-fired thermo-electric power stations (TPSs) in Spain, Bulgaria, The Netherlands, Italy, Turkey, and Greece were used for that purpose. The chemical FA classification system was organized according to the contents, common geochernical associations, and significant positive or negative correlations of ash-forming elements in FAs using three end members, namely (1) sum of Si, Al, K, Ti and P oxides; (2) sum of Ca, Mg, S, Na and Mn oxides; and (3) Fe oxide. This approach divided four chemical FA types (Sialic, Calsialic, Ferrisialic, and Ferricalsialic) with three dominant tendencies (high acid, medium acid, and low acid). The most important phase-mineral FA classification system was organized according to the contents, associations, correlations, properties, and behaviours of species in FAs using also three end members, namely: (1) glass; (2) quartz + mullite; and (3) other minerals. This approach divided four phase-mineral FA types (Pozzolanic, Inert, Active, and Mixed) with three dominant tendencies (high pozzolanic, medium pozzolanic, and low pozzolanic). The specified chemical and phase-mineral FA types and subtypes were characterized and the relationships and distinctions between them were also described. It was found that characteristics such as (1) feed coal and combustion technology used in TPS; (2) water-soluble, magnetic and heavy fractions, pH, fluid ash-fusion temperature, detrital/authigenic index, and BET specific surface area of FAs; and especially (3) content, modes of occurrence, and distribution of glass, quartz, mullite, lime-portlandite, periclase-brucite, Ca sulphates, Ca and Ca-Mg silicates, magnetite-hematite, and char types in FAs; give the most valuable information for the determination of the potential utilization directions and environmental concerns of FAs. (c) 2006 Elsevier Ltd. All rights reserved.