Mitigation measures are needed to prevent large loads of phosphate originating in agriculture from reaching surface waters. Iron-coated sand (ICS) is a residual product from drinking water production. ICS has a high phosphate adsorption capacity and can be placed around tile drains taking no extra space which increases the farmers’ acceptance. The main concern regarding the use of ICS filters below groundwater level is that limited oxygen supply and high organic matter concentrations may lead to the reduction and dissolution of iron (hydr)oxides present and the release of previously adsorbed phosphate. This study aimed to investigate phosphate adsorption on ICS at the onset of iron reduction. First, it was investigated whether simultaneous metal reduction and phosphate adsorption were relevant at two field sites in the Netherlands that use ICS filters around tile drains. Second, the onset of microbially mediated reduction of ICS in drainage water was mimicked in complementary laboratory microcosm experiments by varying the intensity of reduction through controlling the oxygen availability and the concentration of degradable organic matter. After 3 years, ICS filters in the field removed phosphorus under low redox conditions. Over 45 days, the microbial reduction of manganese and iron oxides did not lead to phosphate release confirming field observations. Electron microscopy and X-ray absorption spectroscopy did not evince systematic structural or compositional changes, only under strongly reducing conditions did iron sulfides formed in small percentages in the outer layer of the iron coating. Our results suggest that detrimental effects only become relevant after long operation periods.