Our earlier investigations have demonstrated that the mineral matter of oil shales considerably influences the yield and composition of their thermal decomposition products. However, up to now there are no data about the effect of the dispersion degree of mineral additives on the thermolysis process. Using model mixtures with calcium carbonate and silicon dioxide chemically precipitated on the surface of kerogen concentrate (Estonian kukersite) particles, it has been established that high-dispersed CaCO3 and SiO2 inluence the yield and composition of semicoking products to a notable higher degree than ground, relatively coarce-grained minerals.