It has been pointed out that oxygen contamination of ZnS:Ag:Cl blue phosphor powder mainly comes from the air trapped in inevitably existing pores in heating mixture in crucible. Calculated amount of the trapped air is 2 x 10(-3) Mol per mole a-ZnS in crucible. It is found that addition of AlCl3 (2 x 10(-3) mole) eradicates the oxygen gas in heating mixture, by means of formation of Al2O3 solid. Segregating Na2S4 carries the fine Al2O3 particles to the place that Al2O3 converts to Al2S3 by the concentrated Na2S4. The segregation of Na2S4 requires a slow cooling from 950 to 850 degrees C. The segregation produces the large yellowish particles (> 50 mu m), which are confirmed as ZnO microcrystals surprisingly. Then, we have proposed a model of the growth of ZnO microcrystals in heated mixture which is a sulfur-rich atmosphere. (c) 2005 Elsevier B.V. All rights reserved.