Spain, the main producer of olive oil in the world, generates a big amount of waste from the olive mills. For the treatment of the two-phases Olive Mill Solid Waste (OMSW), which is obtained in higher proportion in these installations, there are different possibilities, among them, one consists of a differentiated treatment of the waste obtaining as final products, three solid fractions such as olive kernel, pulp and residual olive pomace, and a semiliquid fraction, Concentrated Olive Mill Waste Water (COMWW). This last waste, though presenting a lower contaminating effect than the conventional Olive Mill Waste Water (OMWW), has a high percentage of sodium and remarkable levels of COD and BOD that limit its dumping. For the disposal-use of all these by-products, thermal treatment is the most viable option. For that reason, the main objective of this work is to study the viability, optimum conditions and emissions from the combustion of a fuel mixture consisting of solid waste and different proportions of semisolid waste. (c) 2006 Elsevier Ltd. All rights reserved.