To investigate the variation of true density and the sulfur removal forms during the ultrahigh-thermal desulfurization process, two types of delayed petroleum cokes with different sulfur contents were calcined at temperatures up to 2500 degrees C at two distinct heating rates. The influence of the desulfurization coefficient on the true density growth was quantitatively discussed. Additionally, the removal forms of sulfur in petroleum cokes during thermal desulfurization were also investigated through a series of thermodynamic calculations. The results revealed a continuous increase in the true density with the uninterrupted release of sulfur during thermal desulfurization, and a maximum true density of 2.3 g/cm(3) was obtained at a temperature of 2500 degrees C. The growth rate of the true density was dependent on the desulfurization coefficient of the petroleum cokes. The relationship between the true density growth rate and the desulfurization coefficient during thermal desulfurization was an exponential function, which can be presented as YTD = -117 X 10(-5) exp(-10(4)beta(D-s)/1.45) + 9.07 X 10(-5), Furthermore, the calculated results indicated that gaseous elemental sulfur (S-2) was released by the pyrogenic decomposition of ferrous disulfide into inorganic sulfur. Organic sulfur was initially degraded into H2S and SO2 through thiophene decomposition, and then free COS, S-2 and CS2 were released via a carbon reduction reaction.