Results are presented of optical emission spectroscopy (OES) application as a control tool to improve fly-ash plasma vitrification. A twin-torch plasma system has been used for the fly-ash processing and a new OES method has examined metallic vapors above the melt. The method allows the study of nonhomogeneous optically thin plasmas exhibiting a symmetry plane without sophisticated tomographic systems. The de are torches are mounted above a cold crucible filled with a synthetic glass. The arc intensity is from 200 to 400 Angstrom. Argon is introduced into the torches along the cathode and the anode, while argon, oxygen or hydrogen are injected through the lance between the torches. Local plasma temperatures above the melt have been evaluated using measured relative intensities of spectral lines of the plasma-forming gas. Metallic vapor concentration in the plasma is deduced fr om the intensity ratio of the metal-gas spectral lines. Lead oxide has been used to study heavy-metal behavior at the fly-ash plasma vitrification. Distribution of the lead along the crucible surface, depending on the plasma-forming gas composition as well as the concentration evolution with time, have been examined. The elemental analysis of the resultant glass has been measured by scanning electron microscopy (SEM) with energy-dispersive spectrometry(EDS). A predictive model has been adapted to simulate the noncongruent vaporization of heavy metals from the melt. According to the data obtained, steep variations of the volatility of the elements depend strongly on reducing properties of gases controlling the plasma composition near the melted surface. In addition, the melt temperature and the redox potential of the gas phase are found to be the most critical parameters.