Straw of various types of rape, wheat and barley have been studied with respect to the formation of crystalline compounds and high-temperature reactions in ash, as well as sintering and melting behaviour. During the low-temperature ashing process simple, crystalline compounds such as carbonates, sulphates and chlorides were formed. A significant part of the ash from wheat and barley straw was amorphous whereas rape ash was found to be mainly crystalline. The large content of potassium compounds present in wheat and barley straw ash contributes to their low melting points. The ash components primarily formed are reactive. Solid state reactions at temperatures above 800 degrees C lead to the formation of secondary products such as oxides and silicates. Minerals such as kaolin and dolomite have been suggested as fuel additives to give the ash a higher melting point. High-temperature reactions between straw ash and kaolin, Al2Si2O5(OH)(4), or dolomite, CaMg(CO3)(2), respectively, were therefore investigated. Kaolin was found to be the more effective additive. The reaction between kaolin and potassium salts in straw ash gave KAlSiO4 and KAlSi2O6. A laboratory study of reactions involving K2SO4 or KCl and kaolin showed that several products are possible, one of which is KAlSiO4. The potassium capture by kaolin partly explains the higher melting point of the ash-additive mixture. Dolomite added to wheat and barley ash reacted with silica to form silicates. No reaction between dolomite and potassium compounds could be detected. The observed enhancement of the melting point caused by dolomite is probably an effect of dilution or adsorption.