Preventing accidental explosions of flammable liquid/gas mixtures is very important. As far as flammability characteristics are concerned, we simulated the effects of inert liquid/gas, which was filled with reactors, vessels, or closed space, employed in the chemical process industries. The inert liquid/gas (H2O) weakened the oxygen concentration and reduced solvent vapor concentration in a 20-L-Apparatus. This study investigated the flammability characteristics of acetone/water solutions (100/0, 75/25, 50/50, and 25/75 vol.%) that are controlled at a temperature of 150°C and pressures of 101/202 kPa, respectively. The flammability parameters included flammability limits (LEL and UEL), maximum explosion pressure (Pmax), maximum explosion pressure rise ((dP dt−1)max), and vapor deflagration index (Kg). The results of a series of experimental tests showed that UEL, Pmax, and Kg all decreased with steam rising under the experimental conditions. The results can be applied to process safety design/operation for identifying whether the inert liquid/gas (H2O) content has any substantial effects in reducing the fire and explosion hazard of the solution of interest.