The PhD thesis is based on the developed and adapted specific analytical methods for determination of vanadium redox species concentration in the water samples and bioavailable phase of surface sediment of Krka River esturay. The results show that V is predominantly present in the form of V(+V) species in the oxic water column of Krka River estuary, while determined presence of V(+IV) was linked with oxygen depletion in the water layer at the head of estuary, and stabilisation with organic ligands in the anthropogenic burdened sampling site. Determined slight V enrichment is mostly accompanied with retention of V to less mobile sediment phases, possibly due to the low mobility of reduced species and their binding to organic matter. Redox speciation in the bioavailable fraction of surface sediments of Krka River estuary showed that V is present in the form of V(+IV), thus it is not presenting toxic risk for sorrounding biota. In addition, stability of V(+IV)/V(+V) species was investigated in the presence of structurally simple organic ligand (succinic acid) and sulphurised organic ligands (3 mercaptopropionic acid, L-cysteine, thioacetic and ethanthiol) under modelled conditions. Stability constants for V(+IV) and V(+V) complexes with succinic acid in acidic aqueous solution are determined, where higher affinity of V(+IV) species, compared to V(+V), towards complexation was observed. In addition, stability of V(+V) species in the presence of sulpfurized organic ligands was monitored by novel combined approach based on the usage of spectrophotometric and chromatographic instrumentation. Results show that V(+V) reduction is a pH dependant and proton catalysed process which can play a significant role in decreasing V(+V) toxicity in natural systems.