Continuous-flow reactors are widely employed for photochemical reactions due to their numerous advantages, including small dimensions and enhanced process control. The merging of a liquid reaction mixture with a gas phase in these reactors to form gas-liquid systems has significantly expanded the options for synthetic organic chemists, providing a means to mitigate gas-liquid mass transfer limitations. Gaseous reagents offer distinct benefits, being atom-efficient, cost-effective, and simplifying downstream separation processes. These advantages are demonstrated in various photochemical applications such as singlet oxygen chemistry, carbonylations, and light alkane activation. Additionally, inert gases are sometimes introduced to create segmented flow, improving mixing in the liquid phase. This approach has been shown to enhance residence time distribution properties and stabilize solid suspensions. In general, the use of gas-liquid systems in continuous-flow reactors has been shown to improve the performance of photochemical transformations. However, the incorporation of such multiphasic systems introduces additional complexity to photon absorption, as photons interact with gas-liquid interfaces. Small gas bubbles present in low volumetric quantities have been observed to have a minimal effect on photon absorption. Nonetheless, various characterization methods have reported different phenomena in intensified reactors, leading to a lack of consensus on the overall impact. The amount of photons absorbed in a photochemical system is a critical metric for assessing quantum yield, scaling up reactions, and designing energy-efficient reactors. However, accurately determining photon absorption in intensified gas-liquid systems presents a challenge due to the complexity of quantifying the fraction of unabsorbed, transmitted photons, which is necessary to in turn calculate the absorbed fraction from the incident photons. This challenge could be addressed by employing the radiative transfer equation or ray-tracing simulations tailored to the system's specific geometry. Yet, these methods require precise data on bubble size and position, which are not always readily available without extensive dedicated studies. A promising alternative to this complex geometrical determination is the use of the effective optical path length as a one-dimensional parameter to represent complex light interactions. This approach enables reliable assessment of transmitted photons in intensified gas-liquid systems and facilitates the characterization of various photoreactors under different hydrodynamic conditions. For any photoreactor system, illustrated in Figure , the effective optical path length can be determined by solving the mole balance. By selecting an appropriate actinometer, the corresponding balance can be constructed, with an ideal plug flow reactor represented by Equation . The key parameters in this balance include the concentration of the actinometer (𝐶), the liquid flow rate (Φ 𝑙 , controlled by the pump), the photon flux (𝑞, determined by radiometry and ray tracing ), the quantum yield (𝜑, specific to the reaction system), the Napierian absorption coefficient (𝜅, measurable by UV-VIS spectroscopy) and the effective optical path length (𝑙 ̅ , derived from actinometric experiments). In constructing the balance, gas bubbles are assumed to be non-absorbing and wetted at all times, resulting in the absence of the gas flow rate (Φ 𝑔 ) and liquid residence time (𝜏 𝑙 ). Instead, the ratio of photon flux to inlet flow rate is used rather than liquid residence time. These assumptions imply that the photon flux incident on the reactor remains constant, with variations in photon absorption attributed solely to changes in the effective optical path length. In this work, the impact of the gas phase on photon absorption in gasliquid photochemical transformations is investigated using both a microcapillary reactor and a highshear reactor. 𝑑𝐶 𝑑 1 Φ 𝑙 = -𝑞𝜑(1 -exp(-𝜅𝐶𝑙 ̅ )) 1