Fireside corrosion of heat transfer elements in biomass combustion has been widely studied. Several parameters have been detected to contribute the material loss of heat transfer surfaces. The role of fuel is the most significant, but it has to be kept in mind that if sufficiently high steam parameters are selected, corrosion can take place as oxidation even with the best solid fuels available. With proper selection of steel, corrosion rates can be decreased, but it is not possible to totally prevent corrosion reactions. One of the specific aspects in corrosion found in biomass combustion is the presence of deposits. The cases in which corrosion is only due to gas-to-solid reactions are rare, because the concentrations of harmful substances seldom reach levels that are sufficient to allow mass transfer rates required for significant degradation of steel. When a deposit is formed, the composition of the deposit depends on several factors, such as the temperature and the composition of the flue gas, aerosols, and small particles. Depending on the deposit composition close to the metal surface, there may be a risk of partially molten components in the deposit, which may cause rapid corrosion reactions due to the high mass-transfer rates solid-liquid reactions allow. These molten components can be different forms of salts and their mixtures. Due to the nature of the high temperature corrosion, it is very difficult to predict when the melt is formed, and thus the corrosion may initiate when process conditions change. It has been noted that in biomass combustion applications, superheater corrosion may become a problem, if the metal temperature is over 400degreesC. At this temperature region, we find an interesting component, namely potassium hydroxide (KOH). In wood and other biomass-based fuels, the amount of potassium is normally high. The amount of potassium may play an important role in the corrosion of heat transfer surfaces located in the flue gas path, especially in the superheater region. There are strong indications that KOH takes part in the fouling and corrosion of the biomass fired boilers. Operational experiences from the biomass boilers and electrochemical probe measurements indicate that the role of alkali metal hydroxides have been needlessly neglected. The thermodynamic instability of the alkali metal hydroxides in the high vapour pressure of CO2 atmosphere is relevant only when the flue gas temperature is below 700hdegreesC. The temperature profile of a typical biomass boiler is such that the flue gas entering the convective superheater region has a temperature around 850degreesC, and it contains alkali metal hydroxides that condense on the tube surfaces. In coal, oil and peat fired boilers the alkali metal hydroxides are converted to sulphates and chlorides, because these fuels do not contain excess of alkali metals like biomass typically does. In the fouling and corrosion risk evaluation of the fuel, the excess of alkali metals gives a better indication than the content of Cl or S. Reason for this are the alkali metal hydroxides present in the process.