Many technical high temperature processes including coal conversion face problems of alkali vaporization and subsequent condensation of alkali-rich phases. Most alkali-rich phases cause hazardous reactions with a subsequent corrosion of high temperature refractories or construction materials. A reduction of the alkali emission to flue gases would improve the durability of these materials. The alkali retention of coal slag in varying gas atmospheres was studied under controlled gas atmospheres. Samples of ashed coal were treated with gas atmospheres In the range from air to pure CO2 for shea time intervals of 45-50 min. The residue of the experiments was a homogeneous slag that was quenched. The chemical composition of the experimental slag was quantified using wavelength dispersive electron probe microanalysis. The experiments showed that the concentrations of Si, Ca and Fe in the slag were not affected by varying gas atmospheres and temperature. Alkali concentrations show a strong dependence on temperature. The potassium loss increases by 25% over a temperature range from 1350 degreesC to 1450 degreesC. The alkali retention is influenced by the gas atmosphere. The highest retention of alkalis is achieved in pure air. This effect can be attributed to an increased polymerization of the siliceous slag by ferrous iron. At pure CO2 atmospheres the alkali retention displays a second maximum. This effect is most probably due to an accelerated melting already during the heating process of the ashed coal. The rapid melting decreases the time for vaporization and improves the fixing of alkalis into the slag. The results have iplications for high temperature processes in coal conversion, ceramics industry and glass industry. 02000 Elsevier Science B.V. All rights reserved.