The mechanism of formation of Y4Al2O9 during spray pyrolysis of Y2O3:Eu3+ at 1800degreesC was studied by calculating the equilibrium composition in the furnace using a thermodynamic database. At high temperature in the presence of water, Y4Al2O9 was generated from gaseous aluminum compounds volatilized from the alumina reactor tube and yttrium compounds, which were the raw materials of the phosphor. The calculation also indicated that reduction of the amount of water in the furnace would decrease the amounts of volatilized aluminum compounds. Decreasing the amount of water using a diffusion drier controlled the generation of impurities. (C) 2004 The Electrochemical Society.