Inhibition of viscosity increase and fouling in hydrocarbon streams including unsaturation

A method of inhibiting fouling and viscosity increase in hydrocarbon streams including ethylenically unsaturated monomers is disclosed. The method includes the step of adding to the hydrocarbon stream an effective amount of one or more quinone methides of the formula: ##STR1##

wherein R¹, R², and R³ are independently selected from the group consisting of H, -OH, -SH, -NH₂, alkyl, cycloalkyl, heterocyclo, and aryl.