: The effect of solar radiation on the bioavailability of dissolved organic matter (DOM) was investigated during a cruise in the Southern Ocean (similar to 44.5 degreesS, 63 degreesE) in January and February 1999. Seawater was collected in the chlorophyll maximum layer (CML, 30 to 80 m depth) and in deep waters (DW, 200 to 5000 m depth), filtered through 0.2 pm pore-size filters, and exposed to surface solar irradiation for 8 h. The bisavailability of the irradiated DOM and of that in the corresponding dark treatment was subsequently tested in bioassay experiments performed in the dark. Seawater from the CML was also size-fractionated and the effect of solar radiation on the bioavailability of the <20 kDa DOM fraction was compared with that of the bulk DOM. Our results clearly demonstrate that the biological reactivity of DOM? from both the CML and DMI following exposure to solar radiation is inversely related to the initial bioavailability of the DOM Frier to its exposure. The photochemically induced stimulation of the bacterial activity was 2-fold higher in the <20 kDa fraction than in the bulk DOM. These results clearly indicate that initially bioreactive DOM can be rendered biologically more recalcitrant by exposure to solar radiation, while DOM of initially low bioavailability can be, at least partly, photochemically transformed to compounds of higher biological reactivity.