Abstract Aqueous solutions of humic substances (humic and fulvic acids) were irradiated with light at λ = 254 nm followed by analyses of total organic carbon (TOC), ultraviolet (UV) absorbance and molecular weight distribution. A degradation of the dissolved humic compounds by up to 50% after 6 h was observed. The average molecular weight decreased (from 1800 to approx. 300) as well as the UV absorbance of the remaining organic fraction. Low molecular weight organic acids were formed during the irradiation and tentatively identified by capillary electrophoresis (formic, acetic, succinic, and oxalic acids). The low molecular organic acids continued to degrade by further irradiation; the loss of TOC is assumed to be due to CO 2 formation. Indications of a large-scale photo degradation process in nature was observed as a seasonal variation of the ratio humic substances/TOC related to the length of the day. Weekly sampling and analysis of UV absorbance and TOC in the Kalix River in northern Sweden indicate a higher humic fraction of the TOC during winter (dark period) than during summer.