This study investigated the effect of NaOH on the vitrification of electroplating sludge. Ni, the major metal in the electroplating sludge, is the target for recovery in the vitrification. Sludge and encapsulation materials (dolomite, limestone, and cullet) were mixed and various amounts of NaOH were added to serve as a glass modifier and a flux. A vitrification process at 1450 degrees C separated the molten specimens into slag and ingot. The composition, crystalline characteristics, and leaching characteristics of samples were measured. The results indicate that the recovery of Ni is optimal with a 10% NaOH mass ratio:the recoveries of Fe, Cr, Zn, Cu, and Mn all exhibited similar trends. The results of the toxicity characteristic leaching procedure (TCLP) show that leaching characteristics of the slag meet the requirements of regulation in Taiwan. In addition, a semi-quantitative X-ray diffraction analysis revealed that the main crystalline phase of slag changed from Ca-3(Si3O9) to Na4Ca4(Si6O18) with a NaOH mass ratio of over 15%, because the Ca2+ ions were replaced with Na+ ions during the vitrification process. Na4Ca4(Si6O18), a complex mineral which hinders the mobility of metals, accounts for the decrease of metal recovery. (C) 2010 Elsevier B.V. All rights reserved.