In biomass pyrolysis reactions, the wide range of products obtained (some of them environmentally harmful) usually depends on the temperature and the content of amino acids, this fact results in general interest in studying the involved mechanisms of reaction which, in many cases, are poorly understood. In this work, the mechanism of formation of 2,5-diketopiperazine upon pyrolysis of proline at 300°C was theoretically studied by means of DFT calculations (ωB97XD/6-311G(d,p)). All transition states and minimum energy structures involved were optimised, and thermodynamic parameters were theoretically estimated. A mechanism of four steps was proposed, and the most influential step was analysed by using the reaction force formalism, and an analysis of geometrical parameters, charge distributions and bond orders. Results show that the transition state correspond to a dehydration process, and its preceding intermediate, are the most important rate-controlling states of the overall reaction. Reaction force analysis shows a high dominance of geometrical rearrangements (75% of the activation energy), while charge transfer was not an important factor for the analysed reaction step. Finally, from the bond order analysis, it was determined that the rate-controlling transition state is early formed and that the process is asynchronous. (Figure presented.).