Corrosion of superheater tubes is a serious problem during combustion of fuels with a high content of chlorine, such as waste and certain biomasses. The alkali chlorides are released to the flue gas and may condense on the heat exchanger tubes forming corrosive, chloride-rich deposits. In this work the effect of ammonium sulphate ((NH4)2SO4) injection on gaseous alkali chlorides, deposit chemistry and initial corrosion attack of superheater tubes during biomass combustion have been investigated. The investigation was carried out at three different sulphate injection rates (reference, low and high) and at three air excess ratios (λ = 1.1, 1.2 and 1.4). Short-term exposures of Sanicro 28 specimens, using temperature controlled probes, were used for deposit collection and to study the initial corrosion attack. The results showed reduced concentrations of potassium chloride in the flue gas when injecting ammonium sulphate and in particular in combination with high air excess ratios. A decrease of chlorine was also observed in the deposit, but the concentrations did not always correlate to the flue gas measurements. In particular, it was evident that a subsequent sulphation occurred on the tube surface at low air excess ratios. Metallography revealed that sulphation reactions on the tube surface, rather than in the flue gas, had a detrimental effect on the initial corrosion attack. As a consequence, the corrosion attack correlated more clearly to the KCl(g) concentration in the flue gas than the chlorine content in the deposit.