The microstructure of coke has an important influence on its thermal properties. The solution loss reactions of coke in CO2 and H2O atmospheres were investigated by in situ observation. The results showed that the isotropic components had a more vigorous reaction than the anisotropic components, and the solution loss reaction of the fine-grained mosaic structure was faster than that of the coarse-grained mosaic structure under the CO2 and H2O atmospheres. The coarse-grained mosaic structure and the flowing structure had a relatively higher anti-erosion ability in the CO2 atmosphere than in the H2O atmosphere, and there was no distinct difference in the solution loss of the isotropic structure under the CO2 and H2O atmospheres. The electron probe microanalysis showed that the Al–Si–Fe compounds in the carbon matrix had positive influence on the solution loss reaction of the anisotropic structure. The iron compounds were able to destroy the pore walls of coke and accelerate the solution loss rate of coke.