As first part of extensive study on the relation of co-combustion and fly ash quality, co-combustion fly ash was generated from Paso Diablo coal and one secondary fuel, namely poultry dung, demolition wood or solid recovered fuels. The co-combustion experiments were performed in a 1 MWth test boiler (dry bottom). The fly ash was investigated in relation to its performance for use in concrete. It was shown that the properties of co-combustion fly ash can be explained by the characteristics of the fuel and the combustion process. Further it was shown that also fly ash obtained from very high co-combustion percentages (up to 33% e/e) is able to meet the basic requirements of the European standard (NEN-EN 450). Whether an individual co-combustion fly ash does so, depends on the nature of the co-fired fuel and especially on the amount and nature of its inorganic matter. (C) 2012 Elsevier B.V. All rights reserved.