Many accidents that occur in the chemical processing industries such as thermal runaways are due to unstable reactive systems. A good understanding of these systems requires knowledge of the stoichiometries of the reactions that lead to instabilities, which are typically decompositions and/or polymerizations. A screening test to elucidate these stoichiometries is presented. It eliminates implausible stoichiometries from a candidate set due to their thermodynamic infeasibility. We use computational chemistry coupled with statistical thermodynamics to estimate the thermodynamic properties of the unfamiliar species typically involved in such reactions. The real life runaway due to product instability in a process to manufacture tetrahydrofurfuryl benzenesulfonate is treated as a case study. Hartree-Fock and Density Functional Theory calculations are performed to test for thermodynamic feasibility, leading to several postulated stoichiometries corresponding to hazardous thermal decomposition.