The effect of negative corona discharge plasma on the rate of the photocatalytic oxidation (PCO) of acetone and benzene vapors is explored via an in situ IR spectroscopic study of the dynamics of the change in the composition of a gas–vapor system in a 404-L static reactor. The photocatalyst is Hombifine N titanium dioxide, exposed to the light of UV lamps at a wavelength of λ = 365 nm. The rate of the PCO of substrate vapor is compared to those of oxidation in corona discharge plasma, of dark oxidation by ozone (a byproduct of a discharge), of PCO in the presence of ozone, and of PCO upon simultaneous treatment of the medium by corona discharge plasma. It is shown that the rates of oxidation of different substrates, both in plasma alone and in an ozone-containing atmosphere, are much lower than that of PCO. In a corona discharge, however, the rate of PCO increases along with that of the oxidation in an ozone-containing atmosphere. Considerable accumulation of ozone in a gas mixture upon exposure to a discharge is detected only after 80–90% of the oxidized substrate is consumed. The order of the PCO of acetone with respect to ozone is determined. It is shown that plasma treatment substantially accelerates the PCO of benzene by greatly reducing the deactivation of the photocatalyst observed in the PCO of benzene without plasma.