The mathematic model of oil shale thermal decomposition (pyrolysis) was constructed on the base of analysis of available experimental data. The model was applied to the engineering procedure that simulates the Galoter process, namely, pyrolysis of oil shale in a horizontal rotary drum reactor in contact with fine-grained solid heat carrier (hot ash). The model includes the kinetics of organic matter (kerogen) decomposition, the processes of heat and mass transfer inside of single oil shale particle, polydispersity of the particles, their fragmentation, and the secondary chemical reactions such as the carbonization of the emitted shale oil on the ash particles as well as shale oil decomposition the free volume of the reactor. Calculations show that the secondary reactions lead to reduce the shale oil yield from 0.27 to 0.13 kg/kg of dry shale, what is close to operating data of the various industrial units based on the Galoter technology. (C) 2015 Published by Elsevier B.V.