Fly ashes, bottom ashes, and deposits collected on air-cooled probes at a PF-fired boiler cofired with coal and straw (0%, 10%, and 20% straw on an energy basis) have been studied with respect to chemical composition, mineralogy, sintering, and melting. The varying straw share was found not to influence the overall chemical composition of the fly ashes, which were quite alike on an oxide basis, whereas computer-controlled scanning electron microscopy data revealed a change in the species present, meaning that the more potassium that was available for reaction (i.e., the higher the straw share burned), the higher was the fraction of alumino-silicates having reacted to form potassium-alumino-silicates. Comparing compositions of fly ashes and deposits, it was found that K-, Ca-, Fe-rich silicates were concentrated in deposits, probably as an effect of relatively low viscosities of these particles. Based on simultaneous thermal analysis, STA, all ashes examined showed melting in the temperature range from 1000 to 1390 degrees C, and despite the mineralogical differences, no significant difference was found between the melting behavior of the different fly ashes and bottom ashes. When comparing results from the STA melting quantification method to results from the standard ash fusion test, moderate quantities of melt (1-36%) were found at the initial deformation temperature, IDT. Comparing the IDT to the onset of melting as determined by the STA, it was found that the first melting occurred as much as 150 degrees C below the IDT. This stresses that the standard ash fusion method should be used with care when determining melting behavior and thereby ash deposition propensity. Sintering experiments revealed that strength was built up in all ashes at temperatures below the first melt appearance. For the fly ash collected during coal combustion, high strengths were built up in the absence of a liquid phase, whereas for the ashes produced during coal-straw co-combustion, only low strengths were obtained without melt present. On the basis of viscosity calculations it was found that for all ashes the sintering onset was equivalent with an average viscosity of (1-3) x 10(6) P.