Many of the thermal runaway accidents that occur in the pharmaceutical industry are due to exothermic secondary reactions. A good understanding of the runaway (e.g., for dynamic modeling purposes) requires knowledge of the stoichiometries of these reactions. A screening test to elucidate the stoichiometries of secondary reactions 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 secondary reactions. The real life runaway of 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.