A treatment process for municipal solid waste incinerator fly ash has been jointly developed by the Universite Libre in Brussels and the Solvay company. This experimental process has led to an experimental project with a multi-disciplinary team from Solvay along with several companies and organisations, plus support from the ADEME agency. After a laboratory assessment of the treated fly ash (TFA), its environmental properties and the optimal fields of application in road construction, as part of the Revasol project, a full-scale analysis was organised for both environmental and mechanical purposes. A test section using TFA was designed in order to collect percolation water; TFA was used as a road base component (a cement-bound graded aggregate containing 12% TFA). A reference section with the same structure was also built using natural materials. After a description of the treatment process and the test road design, the main TFA characteristics are presented. Then, the environmental behaviour of both sections over a 1-year period following construction is detailed: infiltration, leachate quality, cumulative release. This monitoring effort highlights not only the evolution in TFA behaviour, but also the general permeability of the road structure to various external influences. (c) 2006 Elsevier B.V. All rights reserved.