In an attempt to minimize corrosion in biomass-fired boilers, combustion experiments were performed using binary mixtures of straw with peat, sewage sludge, or grot (branches and treetops). The mixing ratios were carefully selected using literature and thermodynamic calculations. All mixtures were pelletized. The combustion experiments were performed in a laboratory-scale multi-fuel reactor. Extensive analytical analysis of the system included the gas concentration and particle size distribution in the flue gas, the elemental composition of the fuel, and the bottom ash and specific particle size fractions of fly ash. This allowed for the determination of the fate of the main corrosive compounds, in particular, chlorine. The corrosion risk associated with the three fuel mixtures was quite different. Grot was found to be a poor corrosion-reduction additive because of its marginal influence on the chlorine share in aerosols. Grot could not serve as an alternative fuel for co-firing with straw either because no dilution effect on the particle load was measured. Peat was found to reduce the corrosive compounds only at high peat additions (50 wt %). Sewage sludge was the best alternative for corrosion reduction because 10 wt % addition almost eliminated chlorine from the fly ash.