The co-disposal of brine and fly ash has become a common practice in South African power utilities. This study focuses on the effects of the long-term fly ash-brine interaction on the chemical and mineralogical composition of fly ash and the quality of the brine solution after the interaction test. Long-term fly ash-brine interaction test was carried out by contacting fly ash and brine for a period of time varying from 1 week to 12 months under static and closed conditions. The results of the chemical composition of the brine decanted after the interaction test revealed that species such as B, Co, Cu, Pb, Mg, Mn, Zn, Cl and SO4 were removed to a certain extent from brine during the fly ash-brine interaction test while Al, Si, Ca, K, Ba, Sr, Fe, As, Cr and Mo were significantly leached out of the fly ash into the brine. The X-ray fluorescence (XRF) results showed that the concentrations of Na, Mg, Cl and SO4 (as S) in the ash residues were somewhat higher than their concentrations in the fresh fly ash. Secondary mineral phases such as wairakite, charlesite, spinel and celestine which were missing in the X-ray diffraction (XRD) analysis of the fresh fly ash were identified in the ash residues. This study shows that some species contained in the brine solution could be captured by the fly ash through secondary mineralization during co-disposal in a closed static environment, while many other elements could be significantly leached into the brine.