Hollow-core photonic crystal fiber (HC-PCF) offers several advantages for chemical analysis, for example, long optical pathlengths, sub-µL sample volumes, close to 100% overlap between light and sample, and more compact systems for sensing [1–2] and photochemistry [3–4]. Here we compare the performance of a conventional cuvette-based UV-VIS system for monitoring of catalytic reactions with a system based on liquid-filled HC-PCF. We study the photo-Fenton catalytic reaction, which under visible light irradiation induces degradation of methylene blue dye (MB) in the presence of Fe2+ / H2O2 [5].