Cu and Sn ion co-doped phase-separated glasses in R2O-B2O3-SiO2 (R=Na or K) systems were prepared to achieve strong and broad yellow emission under near-UV light irradiation caused by the Cu+ clusters. The Cu+ clusters were formed in the phase-separated glasses, and the resultant broad yellow emission was obtained under near-UV light irradiation. The Cu and Sn ions were concentrated in R2O-B2O3-rich phase. The Cu+ ions became mutually close, thereby forming their clusters. This result agrees with the fact that the emission intensity decreased steeply with the suppression of phase separation because of Al2O3 addition. The emission intensity of the glasses in Na2O-B2O3-SiO2 system was higher than that of the glasses in the K2O-B2O3-SiO2 system because the former glass had large free spaces in the Na2O-B2O3-rich phase attributable to the small ionic radius of Na+ ions. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.001205jes] All rights reserved.