The relative mass of major and trace elements that potentially can be released from the fly ashes generated in the main Greek power plants (Megalopolis A, Amynteon, Agios Dimitrios, Ptolemais, and Kardia) when they are landfilled and exposed to water have been determined. These fly ashes were subject to a one-stage leaching procedure using the Synthetic Groundwater Leaching Procedure (SGLP) test to simulate this situation. The elements Si, Al, Fe, Ti, Ca, Mg, Na, K, S, Ag, As, B, Ba, Be, Bi, Br, Cd, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, Ga, Gd, Ge, Hf, Ho, I, La, Li, Lu, Mn, Mo, Nb, Nd, Ni, Pb, Pr, Rb, Sb, Sc, Se, Sm, Sn, Sr, Ta, Tb, Th, Tl, Tm, U, V, W, Y, Yb, Zn, and Zr have been studied. Results show that the elements with the highest potential hazard for the environment are S, Ca, Br, I, Mo, and Sr. The elements with a moderate mobility are K, Na, Ti, B, Ba, Cd, Cr, Cs, Li, Rb, Sb, Se, Sn, W, and Zn. Si, Al, Fe, Mg, Ag, As, Be, Bi, Ce, Co, Cu, Dy, Er, Eu, Ga, Gd, Ge, Hf, Ho, La, Lu, Mn, Nb, Nd, Ni, Pb, Pr, Sc, Sm, Ta, Tb, Th, Tl, Tm, U, V, Y, Yb, and Zr have a low mobility and, consequently, a low potential hazard for the environment.