This paper presents an investigation into combustion gas composition and deposit formation obtained by co-firing mixtures of cereal co-product (CCP) with a UK coal (Daw Mill) in a 100 kW(th) pilot-scale pulverised fuel (PF) combustion test rig operating at a feed rate of similar to 7.5-11.2 kg/h. The biomass additions to the coal were at 0, 20, 40, 60, 80 and 100 wt.%. A high resolution multi-component Fourier Transform Infra-Red (FTIR) gas analyser was used to monitor the combustion gas stream for CO2, O-2, H2O, SO2, CO, NO, NO2, N2O and HCl. It was observed that increasing the share of CCP in the fuel mixtures resulted in a reduction of SOx and HCl, due to the lower sulphur and chlorine contents of the biomass. Deposit formation in co-firing conditions was also studied on air-cooled probes (with surface temperatures of similar to 500, 600 and 700 degrees C) to simulate the superheater/reheater surface conditions in a conventional pulverised fuel boiler. The deposition flux on the upsteam, sidestream and downstream of the probes were measured. The deposition flux on the upstream surfaces showed a decrease with an increasing biomass content in the fuel. Environmental scanning electron microscopy (ESEM) with energy dispersive X-ray (EDX) analysis and X-ray diffraction (XRD) were used to characterise the deposits. Chlorine was found in all three probes when the CCP was fired without any coal addition, suggesting formation of alkali chlorides through condensation on cooler surfaces (also confirmed by XRD analysis). The study highlights the complex relationship between the fuel mixtures and compositions of the combustion gas streams, as well as deposition fluxes and deposit compositions. (C) 2013 Elsevier Ltd. All rights reserved.