The photocatalytic degradation of the textile dye Reactive Black 5 in distilled and sea water by Ln modified ZnO is studied for the first time under UV-light irradiation. The bleaching process is investigated from different aspects: the type of rare earth element, La3+ concentration and annealing temperature. The observed dye degradation rate increases with the La content up to 2 mol % and then decreases (2.5 and 3.0 mol%). It is found out that Ln modified ZnO photocatalyst achieves contaminants mineralization within a short irradiation time. The optimal dopants concentration and annealing temperature are experimentally established-powders, modified with 2 mol% La3+ and annealed at 100 degrees C, are the most efficient in Reactive Black 5 photodegradation in comparison with pure and (Ce3+, Eu3+) modified ZnO. (C) 2015 Published by Elsevier B.V.