In the Netherlands the government allows the co-combustion of maximum 10% of secondary fuels together with coal. To show whether the quality of fly ashes is affected by co-combustion, a broad research program was performed in which both the technical and the environmental quality of concrete with fly ashes from co-combustion were evaluated in comparison to the quality of concrete with reference fly ashes. In the research program fly ashes from co-combustion of sewage sludge, paper sludge, pet cokes, phosphorous gas, waste wood and a type of liquid hydrocarbon were evaluated. Both the technical and the environmental quality of most ashes from co combustion were shown to be comparable to that of regular fly ashes. This paper reports on the environmental quality of fly ashes from co-combustion; the technical quality is reported elsewhere (Lamers and Van den Berg, 1999). The leaching behaviour of concrete with fly ashes from co-combustion, is diffusion controlled. The components that were potentially introduced because of co-combustion did not lead to increase of leaching. As a result of the outcomes of the technical research program the Dutch regulations for fly ash as a filler or part of the binder were adjusted to make utilisation of fly ashes from co-combustion possible. Commercial operation of co combustion of several secondary fuels is running now at most of the Dutch coal fired power plants.