jats:titleAbstract</jats:title>jats:pHollow‐core photonic crystal fibers (HC‐PCFs) provide a novel approach for in situ UV/Vis spectroscopy with enhanced detection sensitivity. Here, we demonstrate that longer optical path lengths than afforded by conventional cuvette‐based UV/Vis spectroscopy can be used to detect and identify the Cojats:supI</jats:sup> and Cojats:supII</jats:sup> states in hydrogen‐evolving cobaloxime catalysts, with spectral identification aided by comparison with DFT‐simulated spectra. Our findings show that there are two types of signals observed for these molecular catalysts; a transient signal and a steady‐state signal, with the former being assigned to the Cojats:supI</jats:sup> state and the latter being assigned to the Cojats:supII</jats:sup> state. These observations lend support to a unimolecular pathway, rather than a bimolecular pathway, for hydrogen evolution. This study highlights the utility of fiber‐based microreactors for understanding these and a much wider range of homogeneous photocatalytic systems in the future.</jats:p>