Typical benzene series (benzene, toluene, xylenes), while having important applications in chemical industry, are also threatening industrial safety and daily life due to high flammability. In this work, we measured the flammability limits of benzene, toluene, xylenes by ASHRAE method based on E681-09 at elevated temperatures (373 K, 423 K and 473 K). As the temperature increased, the flammable concentration ranges of benzene series expanded. On the basis of flammability limits experimental results, the flame-retardant effect of steam (H2O) on benzene, toluene, o-xylene, m-xylene, and p-xylene were investigated at 423 K separately. These experimental data were correlated with the extended Le Chatelier's law and compared with theoretical estimations based on adiabatic flame temperature (AFT) method. The experimental results were completely reproduced by the extended Le Chatelier's law. The lower and upper flammability limits (LFLs and UFLs) of benzene/toluene + H2O agreed well with the estimations, and the upper flammability limits of xylenes + H2O were within acceptable absolute deviation range. The average absolute deviations (AADs) of the lower and upper flammability limits for benzene series + steam between experimental results and optimized estimations were 0.07 % and 0.38 % separately. Moreover, flame-retardant effect of other diluents (N-2 and CO2) on benzene series were calculated theoretically, and all the critical flammable ratios (CFRs) were close to 1. The results showed that the flame retardancy on benzene series need to be considered seriously rather than using common diluents. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.