Fly ash sampling carried out in a large combustion unit and in the chimney of a private house indicates that the composition of fly ash is significantly different from the original coal composition. Based on their mass-size distributions, the elements can be divided into three groups. A first group of elements is associated with large particles and is depleted relative to the original coal composition. A second group has a maximum concentration of small particles and is enriched in fly ash. For a third group, the mass-size functions show an intermediate behaviour. Thier concentration ratios in fly ash and coal are near unity. This division into enriched and depleted elements appears to be roughly independent of the operating conditions of the combustion process. There is some indication that the specific mass-size distribution in fly ash emitted by small domestic facilities is different from that in fly ash from larger facilities.