This work presents results of an in-depth experimental microstructural characterisation of P91 steel, following exposure to salts representative of ash produced during biomass co-firing. The ash produced during co-firing contains high levels of harmful elements such as potassium, sodium and chlorine, which result in the accelerated corrosion of heat exchanger tubes in fluidized bed power plants. Synthetic salts representative of 70:30 peat-biomass and 40:60 peat-willow mixtures have been produced and applied to samples at 600 degrees C in air for up to 28 days. Samples have been etched to reveal thesteel microstructure, allowing for a comprehensive evaluation of microstructural degradation of the samples. Cracks associated with the delamination of scales have been identified at the oxide-metal interface, with multiple layers of oxides forming as a result. A number of pits have been identified in the samples, which are associated with inclusions located close to the sample surface. Finite element analysis of the effect of differential thermal expansion coefficients between the inclusions and base metal shows the detrimental effect of such inclusions, leading to stress concentrations at the inclusion-metal interface. (C) 2016 Elsevier Ltd. All rights reserved.