Jet loop reactors (JLRs) are widely used as multiphase reactors in chemical, biological and environmental processes. They use a strong liquid jet from a nozzle to force internal circulation of both liquid and gas, as well as entrainment and dispersion of the gas phase. This results in excellent mass and heat transfer in a well-mixed system without the need for internal stirrers. Despite their advantages, the complex operating behavior and design of these reactors present significant challenges. It is obvious that the properties of the disperse internal flow are important for the behavior of the JLR. However, little information is available on the spatial variations of these two properties within the JLR. Therefore, we used a needle probe to study the spatial variations of gas fraction and bubble size distribution (BSD) in a DN165 JLR. The JLR was operated with aqueous solutions and air. Process conditions (e.g., power input), reactor geometry (e.g., draft tube diameter), and liquid phase composition were systematically varied. Our results show that strong variations of the BSD and the gas fraction occur within the reactor. Furthermore, the influence of the liquid phase properties becomes evident. When the concentration of electrolytes in the aqueous phase is low, the BSD along the flow path shifts to a larger mean diameter and the BSD becomes wider. This change in BSD indicates strong coalescence in the reactor. As expected, higher electrolyte concentration promotes coalescence and thus the spatial variations of the BSD become much smaller. For the coalescence inhibited systems, the variations of the gas fraction and the BSD account for a difference in the local volumetric interfacial area of up to 80%. Not only the local mass transfer is affected, but also the stability of the internal circulation flow. We define stability as the highest (or lowest) gas hold-up at which a stable internal circulation flow is maintained in the JLR for a given geometry and operating condition (e.g., power input). Our results show that the stability of the internal circulation flow is much higher for the coalescence-inhibited systems than for systems in which coalescence and large bubbles occur. Comparative simulations with a 1D model based on a momentum balance suggest that the reason for this behavior lies in the different slip velocities that occur in systems with small or large bubbles. As a result, in systems with large slip velocities, the difference between the gas holdup in the annular gap and the draft tube increases. The larger this difference, the more the internal circulation will be slowed down until it finally collapses. Our results show that the coalescence and breakage of the gas bubble in the JLR affects not only the interfacial area, but also the stability of the JLR. These phenomena must be taken into account for reliable scale-up and operation of the JLR. [1] Breit, F., Bey, O., & von Harbou, E. (2022). Processes, 10(7), 1297. https://doi.org/10.3390/pr10071297