A novel kinetic approach based on ab initio calculated rate constants has been developed and implemented in the kTSim program. The proposed approach allows prediction of the distribution of reactant and product concentrations over time, based exclusively on computationally obtained rate constants. The newly developed methodology was used to simulate the process of evaporation and tautomerization of guanine and cytosine under thermal (T = 490 K, cytosine; T = 620 K, guanine) and laser (T = 1000 K, 24 ns laser pulse) desorption conditions. Both monomolecular and bimolecular mechanisms of the tautomerization were considered simultaneously. The rates of the reactions were estimated using the values of Gibbs free energies calculated at the MPWB1K/aug-cc-pVDZ level and specified in a kTSim input. We expect that the proposed approach can also be used for accurate kinetic simulation of a wide range of processes.