The kinetics of hydroquinone-inhibited oxidation of acrylic acid and methyl methacrylate was studied volumetrically by measuring the oxygen uptake in the presence of an initiator (azobisisobutyronitrile) at T = 333 K and P _{O 2} = 1 and 0.21 atm. The oxidation of acrylic acid inhibited by 4-methoxyphenol was studied under the same conditions for comparison. The rate constants of the reactions of the peroxyl radicals of acrylic acid (∼CH₂CH(COOH)O ₂ ^· ) and methyl methacrylate (∼CH₂CMe(COOMe)O ₂ ^· ) with hydroquinone (HOC₆H₄OH) (1.20 × 10⁵ and 7.16 × 10⁴ l mol⁻¹ s⁻¹, respectively) and of the reaction of peroxyl radicals of acrylic acid with 4-methoxyphenol (p-CH₃OC₆H₄OH) (3.25 × 10⁴ l mol⁻¹ s⁻¹) were measured. The stoichiometric inhibition factors f were determined. The reaction between the semiquinone radical and oxygen, O₂ + HOC₆H₄O^·, plays an important role, decreasing the factor f and the efficiency of the inhibition effect of hydroquinone. The rate constants of this reaction were calculated from kinetic data: k = 5.72 × 10² (in acrylic acid) and 4.60 × 10² l mol⁻¹ s⁻¹ (in methyl methacrylate).