Owing to Denmark's pledge to reduce carbon dioxide emissions, biomass is being increasingly utilised as a fuel for generating energy. Extensive research and development projects, especially in the area of material performance for biomass fired boilers, have been undertaken to make biomass a viable fuel resource. When straw is combusted, potassium chloride and potassium sulphate are present in ash products, which condense on superheater components. This gives rise to specific chlorine corrosion problems not previously encountered in coal fired power plants. The type of corrosion attack can be directly ascribed to the composition of the deposit and the metal surface temperature. In woodchip boilers, a similar corrosion rate and corrosion mechanism has on some occasions been observed. Cofiring of straw (10 and 20% energy basis) with coal has shown corrosion rates lower than those in straw fired plants. With both 10 and 20% straw, no chlorine corrosion was seen. The present paper will describe the results from in situ investigations undertaken in Denmark on high temperature corrosion in biomass fired plants. Results from 100% straw firing, woodchip and cofiring of straw with fossil fuels are summarised and compared.