Influence of dissolved oxygen at different pressure to photocatalytic and photoelectrocatalytic azodye C.I. direct black 22 (DB) oxidation is investigated. It is shown, that at the presence of dissolved oxygen photocatalytic and photoelectrocatalytic DB oxidation rate on TiO₂ increases. The azodye decolouration higher than 98% by photoelectrocatalytic oxidation at oxygen pressure was 0.42 MPa.