Polymerization reactions have caused a number of serious incidents in the past; they are prone to reaction runaways because of their exothermic and autoaccelerating nature. To minimalize the risk, the reaction is commonly performed in a solvent as empirical industrial practice. In this work, the thermal runaway hazards of the ethylbenzene-styrene system with different monomer mass fractions were calorimetrically investigated up to temperatures where decomposition products are unlikely to be produced. Experiments showed that the polymerization runaway "onset" temperature inversely increased with the monomer mass fraction. Experiment and thermodynamic calculations showed that volatile diluent increased system vapor pressure even at a lower adiabatic temperature rise and verified that moderation of the risks could be achieved if the monomer mass fraction is below ca. 85%. A lumped kinetic model developed by Hui and Hamielec was used to predict the runaway profile of this reaction under different dilutions, and the agreement was excellent.