Salts of N-nitrosophenylhydroxylamine

A method for inhibiting formation of undesired polymer from an ethylenically unsaturated monomer, which comprises adding to the monomer an effective amount of a salt of N-nitrosophenylhydroxylamine (NPHA) and an amine, expressed in the neutral form as Formula I:

• ##STR3## wherein n is an integer of from one to the number of basic nitrogens in the amine;
• R.sup.1 is hydrogen or C.sub.1 -C.sub.5 alkyl;
• R.sup.2 is hydrogen, C.sub.1 -C.sub.5 alkyl, or C.sub.1 -C.sub.10 aminoalkyl;
• and R.sup.3 is C.sub.1 -C.sub.5 alkyl or C.sub.1 -C.sub.10 aminoalkyl provided that:
• a. if R.sup.1 is hydrogen then R.sup.2 and R.sup.3 cannot both be C.sub.1 -C.sub.5 alkyl,
• b. if R.sup.2 is hydrogen then R.sup.1 and R.sup.3 cannot both be C.sub.1 -C.sub.5 alkyl, and,
• c. if R.sup.1 and R.sup.2 are both hydrogen then R.sup.3 cannot be C.sub.1 -C.sub.5 alkyl.