A method for the determination of iron(II) in water in the presence of iron(III) with 1,10-phenanthroline is described, in which the interfering effect of iron(III) ions is suppressed by masking with complexones. In the absence of a chelating agent for iron(III) the colour intensity of samples being analysed is unstable, owing to redox processes induced by the effect of 1,10-phenanthroline on the iron(II)-iron(III) system, causing reduction of iron(III) and formation of the corresponding coloured iron(II) chelate compound in a stoicheiometrically proportional concentration. The advantages of complexones, especially nitrilotriacetic acid, over other chelating agents are discussed.