This study investigated the effects of additives on reduction in ash deposit formation while co-firing straw pellets with pulverized coal. A pulverized-fuel-fired combustion system was used to co-combust sub-bituminous coal and herbaceous biomass. Kaolin (Al2Si2O5(OH)4) and ammonium sulfate ((NH4)2SO4) were used to determine the influence of additives on ash deposit reduction. Experiments were performed in an 80 kWth scale combustion system for various proportions of additives (proportions of 1 wt% and 4 wt% of fuel). Weight of deposited ash was measured concurrently. In the experimental results, different trends from several previous researches were observed regarding effectiveness of the additives: Addition of kaolin increased deposition by up to 19%, while ammonium sulfate increased deposition by up to 36%. Thermodynamic equilibrium of all experimental cases was calculated based on ash composition, and we reveal that in our experimental cases kaolin promotes formation of K-silicates of low melting temperature, while ammonium sulfate reacts with KCl to increase melting temperatures for K-Al-silicates. This demonstrates that the fuel-specific choice of additives is important to effectively reduce ash deposition.