Photochemical reactions in the Eu(III)–humic acid system are investigated by fluorescence spectroscopy. For comparison, humic acid without europium is also studied. Irradiation is performed by high energy laser beam and a low pressure mercury lamp. The impact of photodegradation on spectroscopic properties, size and decomposition of humic acid is monitored by steady state and time-resolved fluorescence spectroscopy, UV–Vis spectroscopy, gel permeation chromatography (GPC), dissolved organic carbon (DOC) analysis and ultrafiltration. The different indicators for photodegradation show different sensitivity. The decrease in DOC content with increasing irradiation dose is lower than the decrease in UV/Vis absorption. The highest impact is found for the fluorescence intensity. At 3 kJ/mg humic acid absorbed energy and in absence of europium, fluorescence diminishes by more than 90%. In the presence of Eu(III), however, fluorescent groups are partly stabilized in this range of absorbed energy. Results from GPC show changes in the chemical structure, especially generation of smaller entities. The photodegradation of the humic acid leads to a decrease of the europium–humate complexation constant. Furthermore, europium is reduced to the divalent state. The present study shows that for metal ion humic acid complexation studies by laser fluorescence spectroscopy, great care is needed to avoid significant experimental artifacts, such as photodegradation and metal ion redox reactions.