Polymerization inhibitor compositions are provided. The polymerization inhibitor compositions include at least a first inhibitor compound having a phenothiazine or a derivative thereof, a second inhibitor compound having a phenylenediamine or a derivative thereof, and a solvent. Methods of inhibiting the polymerization of monomers using the compositions of the disclosure are also provided. The methods of inhibiting polymerization of monomers include a step of adding a composition of the disclosure to the monomer. In some instances, the monomer is an ethylenically unsaturated monomer. Such ethylenically unsaturated monomers include, but are not limited to, (meth)acrylic acid, methyl methacrylate, acrylic acid, acrylic acid esters, methacrylamide sulfate, vinyl acetate, acrylonitrile, acrolein, acrylates, methacrylates, 1,3-butadiene, styrene, isoprene, and combinations thereof. Methods of preparing the polymerization inhibitors and compositions of the disclosure are also provided.