The present invention provides substituted phenols which are effective to inhibit polymer formation during the manufacture of acrylonitrile. Preferred substituted phenols are (a) hindered phenols, nonhindered phenols, and partially hindered phenols, or (b) combinations of hindered phenols with nonhindered or partially hindered phenols. A most preferred substituted phenol is p-nitrosophenol. In a preferred embodiment, the substituted phenols (with the exception of p-nitrosophenol) are combined with hydrogen transfer agents.