This paper provides a simple guideline to quantify at what gas fraction and bubble diameter bubbles start having a considerable effect on the performance of a bubbly slurry photocatalytic reactor. The local rate of photon absorption, responsible for local photoreaction rate, is affected in the presence of bubbles, as bubbles scatter the photons in all directions. Here, we consider a simple 1D description of a photoreactor, and implement a bidirectional scattering model for photocatalytic particles and bubbles to set up photon balances in the photoreactor. Both low and high light intensities are considered, with a linear and square root dependence of reaction rate on the local volumetric rate of photon absorption, respectively. The photon balances, coupled with component balances, lead to a closed-form expression for the critical gas fraction that causes a 5% deviation in the photoreaction rate, when compared to an ungassed reactor with the same amount of catalyst per unit reactor volume. We further show that in typical bubbly slurry photoreactors, with gas fractions smaller than 20% and bubble sizes of about 3. mm, the effect of bubbles on the rate of photon absorption and photoreaction is negligible. Moreover, the overall efficiencies are calculated at different gas fractions and bubble diameters. Other operational aspects of three-phase photoreactors are calculated and briefly discussed. The general conclusion is that the distribution of light inside a photoreactor is hardly affected by the presence of bubbles.