4-Hydroxy-4-methyl-2,5-cyclohexadien-1-one has been conclusively identified by its H-1 and C-13 NMR spectra as a significant initial product in the radiolysis of aqueous solutions of p-cresol. This product is formed as the result of oxidation of the hydroxycyclohexadienyl radicals produced by addition of similar to12% of the *OH radicals to the aromatic ring at the ipso position adjacent to the methyl group. It has a strong absorption band at 228 nin that is similar to the 246 nm band of p-benzoquinone. Its proton NMR spectrum exhibits strong coupling between two pairs of ring protons. A quartet is observed in the spectrum of the methyl-C-13 labeled product, confirming that a methyl group is attached to the dienone ring. C-13 chemical shifts and C-13-H spin-spin splittings of the dienone are reported. Although DFr calculations of the proton NMR parameters are in very good agreement with the experimental values, the calculated chemical shifts of the ring C-13 carbons are 4-8 ppm too high, indicating that the DFT calculations do not properly take into account the dienone 17 system. This conclusion is confirmed by parallel experimental and theoretical studies,of 4H-pyran-4-one that provides a model system closely related to the dienone. The observation of 4-hydroxy-4-methyl-2,5-cyclohexadien-1-one as a product in the radiolysis of aqueous solutions is important in demonstrating that addition of *OH radicals to aromatic rings at positions substituted with alkyl groups can be of appreciable importance in the *OH oxidation of aromatic substrates.