steps such as isomerization or alkylation the alkylbenzene column bottoms fraction comprising principally the vinyltoluene precursor ethyltoluene is withdrawn from the bottom of alkylbenzene column 26 through line 32 and returned to the dehydrogenation reactor to produce additional vinyltoluene generally polymerization inhibitors are introduced into the vinyl aromatic monomer during the distillation preferred inhibitors include nitrated phenolic compounds such as dinitro o cresol dinitro p cresol m nitro p cresol dinitrophenol n nitroso diphenylamine 4 halo 3 5 dinitrotoluene 3 nitro 2 5 cresotic acid and the like sulfur may also be used as an inhibitor but its use is not preferred because the resulting sulfur containing tarry residues have little economic value and are very difficult to dispose of mixtures of inhibitors may be used a particularly preferred inhibitor comprises a mixture of n nitroso diphenylamine ndpa which is active primarily in the vapor phase and dinitro p cresol dnpc which is active primarily in the liquid phase the inhibitors may be introduced in any desired manner inhibitor from a source of inhibitor 34 may be introduced through line 36 into the crude vinyltoluene feed in line 2 prior to introduction of the feed into recycle column 4 inhibitor from a source 38 may be introduced directly into recycle column 4 and or finish column 12 through lines 40 and 42 respectively it is also possible to add inhibitor to the reboilers the amount of inhibitor required depends upon the specific inhibitor used but generally lies between about 50 and about 3000 ppm with respect to the vinyltoluene higher amounts may be utilized but ordinarily little benefit is gained from the additional expenditure in.