In Denmark, biomass such as straw or woodchip is utilized increasingly as a fuel for generating energy. When straw is combusted, potassium chloride and potassium sulfate 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. Co-firing 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. This 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 co-firing of straw with coal will be reported. The corrosion mechanisms observed are summarized and the corrosion rates for 18-8 type stainless steels are compared.