Reaction pathways of the light hydrocarbons formation during propane and isobutane dehydrogenation on Al-Cr catalyst have been studied. It was determined that the majority of light compounds are formed not directly from initial paraffin, but from the main product of the paraffin dehydrogenation; olefin. The sequence of the light compounds formation reactions includes: (1) dehydrogenation of the initial paraffin with formation of targeted olefin; (2) consecutive hydrocracking of the newly produced main olefin with formation of the one carbon shorter chain olefin and methane; (3) one carbon shorter chain olefin can be converted by two parallel reactions further. The first reaction is hydrogenation of the one carbon shorter chain olefin to the formation of the one carbon shorter chain paraffin. Another parallel reaction is hydrocracking of the one carbon shorter olefin to the two carbon shorter olefin and methane. If short olefin is ethylene the final product of the hydrocracking reaction is methane. This sequence can be presented by using isobutane dehydrogenation as an example which also includes steps of light compounds formation from C-3 hydrocarbons: C-4 H-10 -> C-4 H-8 + H-2 (1) C-4 H-8 + H-2 -> C-3 H-6 + CH4 (2) C-3 H-6 + H-2 -> C-3 H-8 (3a) C-3 H-6 + H-2 -> C-2 H-4 + CH4 (3b) C-2 H-4 + H-2 -> C-2 H-6 (4a) C-2 H-4 + 2H(2) -> 2CH(4) (4b) Obtained reaction pathways of light compounds formation can be applied to optimize the concept of the dehydrogenation process and explain puzzling phenomena regarding the temperature profile in the fixed bed dehydrogenation process. (c) 2010 Elsevier B.V. All rights reserved.