Y2O3:Eu red-emitting phosphor was prepared by solid-state reaction from Y2O3 center dot Eu2O3 mixed oxide using BaB4O7 and SrF2 concurrent double flux followed by washing treatment. The evolution of fluoride flux added during firing and washing treatment was monitored by the fluoride selective electrode method. The determination of fluoride in the presence of large quantities of rare-earth (RE) elements in matrix was explored. The analysis results using the addition-recovery method showed that the addition of ethylenediaminetetraacetic acid could mask the RE elements, making fluoride determination reasonably accurate. The measurement results revealed that, depending on the concentration of fluoride flux added in the blended mixture and firing temperature, ranging from 20 to 80% of fluoride added, was lost by evaporation during the firing process. About 15% of the residual fluoride in the fired phosphor was washed out when using cold water, while 25 to 70% of fluoride in the fired phosphors was washed out by hot water. Depending on the firing temperature and concentration of fluoride added in the blended mixture, about 13-35% of the added fluoride still remained in the final phosphors after being fired and washed with water two times.