The densities of re-melted coal slag melts were measured in air using Archimedean double-bob method in the temperature range from 1350 to 1600 degrees C. The main components of gasified coal slags are SiO2, Al2O3, CaO, MgO, FeO and Fe2O3. Slags consisting of the main components with systematic composition variations were synthesized and evaluated for comparison. The experimental results of chelate titration analysis revealed that the percentages of Fe3+ in coal and synthesized slag samples were 47-56% and 68-80% of total Fe, respectively. The density decreased linearly with increasing temperature for all melt samples. The density of gasified and synthesized coal slag melts was found to decrease with increasing Al2O3 and SiO2 contents and to increase with increasing Fe2O3 and FeO contents. The molar volume and the coefficient of volume expansion, which were calculated from measured density values, increased monotonically with increasing Al2O3 content for CaO-Al2O3 SiO2 (CAS) synthesized samples with 40 mol% SiO2. The molar volume for CaO-FeO-Fe2O3-SiO2 (CFS) synthesized samples with 60 mol% SiO2 increased with Fe2O3 addition and exhibited discontinuous change at, similar to 7.5 mol% Fe2O3. The coefficient of volume expansion in the same CFS series decreased with increasing Fe2O3 content showing a minimum value at similar to 7 mol% Fe2O3 and increased in higher Fe2O3 content. A composition parameter calculated from chemical composition was proposed to predict the density of coal slag melts. The relationship between measured density and the composition parameter was analyzed. (C) 2016 Elsevier Ltd. All rights reserved.