As an advanced oxidation process, hydrodynamic cavitation generates radicals inducing reduction of chemicals in water. In the present work dye degradation is investigated as a representative for such chemical. Cavitation intensity, outgassing and flow reactivity largely depend on pressure boundary conditions. The paper presents an experimental study aimed to investigate effects of outgassing on degradation through jet cavitation in a multiphase reactor by varying back pressure between 0.6 and 2 bar at a constant pressure difference of 40 bar. The measurements reveal that some outgassed air bubbles are recirculated into the jet, which may enhance the process as an oxidizing agent. Degradation is found to vary significantly by back pressure obtaining maximum degradation around ambient pressure in the experimental setup used. But outgassing also restricts reactivity at back pressures below ambient pressure. The influence of outgassing on degradation unlocks opportunities for energy-to-degradation efficient applications.