Borehole samples of weathered PFA were taken from an ash disposal mound at a major British power station. The oldest ash studied was deposited in 1975. XRD and SEM investigation of weathered PFA showed no substantial changes in mineralogy, such as the formation of secondary clay minerals, as the result of weathering. However, element concentrations in porewater and PFA varied as a function of depth indicating (a) reaction of PFA with infiltrating porewaters and attainment of equilibrium (Ca, SO42-, Ba, St), (b) reaction without attainment of equilibrium (Mg, Na, K, B, Mo) and (c) elements infiltrating into the PFA from the surface (Cl-, NO3-). The depth trends were used to assess mobility and migration of elements in the PFA mound and thus to produce information on leachate evolution in older PFA disposal sites in which leachates could either enter surface waters or pass downwards into groundwater.