Phosphonates are an important group of phosphorus-containing compounds due to their increasing industrial use and possible eutrophication potential. This study involves investigations into the methods UV/Fe-II, Fenton and UV/Fenton for their removal from a pure water matrix and industrial wastewaters. It could be shown that the degradability of phosphonates by UV/Fe-II (6 kWh/m(3)) in pure water crucially depended on the pH and was higher the less phosphonate groups a phosphonate contains. The UV/Fell method is recommended in particular for the treatment of concentrates with nitrogen-free phosphonates, only little turbidity and a low content of organic compounds. Using Fenton reagent, the degradation of polyphosphonates was relatively weak in a pure water matrix (<20% transformation to o-PO23-). By means of the Photo-Fenton method (6 kWh/m3), those phosphonates with the smallest numbers of phosphonate groups were easier degraded as well at pH 3.5 in a pure water matrix (o-PO43- formation rates of up to 80%). Despite an incomplete transformation of organically bound phosphorus to o-Par with Fenton reagent in an organically highly polluted wastewater (max. 15%), an almost total removal of the total P occurred. The most efficient total P elimination rates were achieved in accordance with the following Fenton implementation: reaction -> sludge separation (acidic) -> neutralization of the supernatant -> sludge separation (neutral). Accordingly, a neutralization directly after the reaction phase led to a lower total P removal extent. (C) 2017 Elsevier Ltd. All rights reserved.