The purpose of the study described in this paper was to determine the removal of Cr (total), Cr (VI), Cu, Ni, Pb, Zn and Cd from wastewater using different particle-size-fractions of highly-calcareous and highly-siliceous fly ashes (FAs). Three different Hellenic FAs (2 calcareous and 1 siliceous) were tested for their capability of precipitating heavy metals from aqueous solutions. Each FA sample was separated into 6 different size-fractions with a grain diameter range of: [(0-25) (25-40) (40-90) (90-150) (150-400) and (>400)] ým. The different FA grain fractions were evaluated in terms of their chemical composition, pH, loss on ignition (LOI) and CaOf (%). Batch adsorption experiments were then carried out, indicating that the various grain fractions of the highly siliceous FA were more efficient in precipitating Cr (VI) but less capable of retaining Cd, Cu, Ni, Pb and Zn. On the other hand, the high-Ca fly ashes were proven to be more efficient in uptaking Cd, Cu, Ni, Pb and Zn, but less in hexavalent chromium. This particular tendency was also confirmed in the case of the different particle-size-fractions of same fly ashes. It was actually verified that FAs can be more effective in the field of industrial wastewater-remediation when separated into their size-fractions.