Municipal solid waste incinerator (MSWI) bottom ash is predominantly composed of high temperature solids. In a natural atmospheric environment many of these solids are metastable and will alter to form thermodynamically stable assemblages of minerals. Results of our research have revealed that the weathering products of MSWI bottom ash, which has been disposed of in the open, are similar to those found in weathered volcanic ashes and scoriae. The most obvious change is the transformation of glassy constituents into clay-like materials. These secondary products may have a significant influence on the leaching characteristics of contaminants such as heavy metals. Although the influence of this alteration process on total leaching is yet to be determined, our results suggest new topics of future research on geochemical engineering concepts, where natural weathering processes are exploited to minimize deleterious effects of ash disposal and utilization.