Quinone methide derivatives such as
4-benzylidene-2,6-di-tert-butyl-cyclohexa-2,5 dienone are used to inhibit
styrene monomer polymerization in the dehydrogenator portion of a styrene
monomer production system. The inhibitor contacts the dehydrogenation
effluent and does not partition in substantial amounts to the aqueous
phase that is separated in the phase separator.