Single-crystal, submicrometer-sized CaS:Eu luminescent particles were synthesized via a solvothermal route, and these moisture-sensitive particles were coated with aluminum oxide using atomic layer deposition (ALD). Photoluminescence (PL) spectra of coated and uncoated particles were compared. They both showed a broad-band PL emission with a maximum of 650 nm. Microencapsulation by aluminum oxide layers did not have a pronounced effect on the intensity of the emission. In situ luminescence measurements during the accelerated aging (80 degrees C, 80% relative humidity) of coated and uncoated CaS: Eu particles were performed. While the uncoated phosphor was largely degraded within 30 h of aging, it was observed that a 20 nm thick aluminum oxide coating dramatically increased the resistance of the luminescent material against moisture, showing the conformity of the Al(2)O(3) coating by the ALD process. Upon degradation, CaCO(3) was formed, leading to Eu(3+) emission as observed in cathodoluminescence. Finally, the use of these coated particles as a wavelength conversion material in light-emitting diodes was evaluated. (C) 2009 The Electrochemical Society. [DOI:10.1149/1.3211959] All rights reserved.