Fluorescence at wavelengths characteristic of humic substances (excitation 350 nm, emission 450 nm) have been used in this study to approximate concentrationsof fluorescent dissolved organic material (FDOM). In situ regulated and unregulated benthic chambers, sediment cores, and laboratory tank incubations were usedto study early diagenesis of FDOM in coastal marine sediments of the Gullmar Fjord, western Sweden. In the regulated in situ chambers, pH and oxygen werekept at relatively stable levels, while in the unregulated in situ chambers, pH and oxygen were left to decrease as a result of biological activity. FDOM porewaterdistributions and correlation between FDOM and parameters indicating mineralization showed that FDOM was formed in the sediment and should flux across thesediment-water interface. A substantial flux of FDOM was also observed during winter and spring conditions and during anoxic conditions fall. However, no fluxwas observed during oxic conditions fall. Modeling indicated that oxygen penetration depth was deeper during winter than during fall, i.e., the oxygen penetrationdepth increased during fall towards winter values. We suggest that as FeOOH was formed when oxygen penetration depths increased, FDOM was sorbed tonewly formed FeOOH, inhibiting FDOM flux over the sediment-water interface. In addition, at onset of anoxic conditions in the sediment surface layer in fallincubations, FDOM flux from sediment to overlying water increased substantially. Increases in anoxic FDOM fluxes were accompanied by increases in Fe andphosphate fluxes. We suggest that reductively dissolved FeOOH released sorbed FDOM. FDOM released from FeOOH by anoxic conditions was not resorbedwhen oxic conditions were resumed. This could be an effect of higher pH in overlying water as compared with porewater, inhibiting FeOOH sorption of FDOM.