First of all, the stability of the PS was tested in DCM:THF, 9:1 (Figure ). The readily available methylene blue, MB, despite its low absorption at the selected wavelength (ε525 ≈ 4000 M -1 ⋅cm -1 ) entirely bleached upon irradiation, while 5, 10, 15, 20-tetrakis(3-hydroxyphenyl)-porphyrin (mTHPP, (ε525 = 10000 M -1 ⋅cm -1 ) was photostable under the same conditions. PABA was exploited as reference substrate for the photooxygenation step being its EPO analogue PABA-O2 readily formed and easy to handle (Figure ). Various combinations of residence times, light intensities and molar ratios between PABA and PS (either MB or mTHPP) were tested in order to optimize the in flow reaction conditions with respect to the previously published procedure based on a 100 W Hg-lamp setup (Figures ). Eventually, the performance of the photochemical step in flow yielded quantitative formation of PABA-O2 reducing 5 times the amount of PS (1% mol/mol) in only 10 minutes, resulting in a 6-fold faster reaction compared to previously reported batch conditions. Moreover, the same performance was achieved by using 80% of the intensity of irradiating light enabling even milder reaction conditions.