The increase in use and extraction of Rare Earth Element (REE) over the last few decades has raised concerns about their behaviour in the environment. One of the factors controlling their fate is adsorption onto iron (Fe) (oxy)hydroxides. In natural systems, aluminium (Al) substitutions of iron often occur in these (oxy)hydroxides, as Al is also one of the most abundant elements at the Earth's surface. Though it has been shown that these substitutions can have an impact on the adsorption of different metallic contaminants, to date, no study has been carried out on the impact of substitution on the adsorption of REE.To fill this gap, a series of ferrihydrites and goethites with varying levels of Al and gallium (Ga) substitution were synthesized, with Ga used as it is a chemical analogue of Al with an ionic radius comparable to that of Fe, unlike Al whose radius is smaller. The synthetised (oxy)hydroxides were subjected to REE adsorption at acidic to near neutral pH. REE complexation modes were studied using EXAFS collected at the neodymium L3-edge. Results show that the amount of adsorbed REE is controlled both by the pH of solution and the structure of the adsorbing (oxy)hydroxide. Adsorbed REE form both bidentate mononuclear (BM) and bidentate binuclear (BB) complexes. Unlike goethite, in the ferrihydrite the substitution affects the size, shape and organisation of the surface sites of the particle. The number of sites allowing BM complexation decrease with increasing substitution, as Al and Ga inhibit the protrusion of metal octahedra at the (1−10) plane. Additionally, Al substitution inhibits the growth of the step wise (1−1−1) plane and its reciprocals, leading to the particle shape changing from spherical to cylindrical and a loss of adsorption sites capable of forming both BM and BB complexes.With this study we demonstrate that the substitution of Fe by Al and Ga in goethite has little to no effect, while in the ferrihydrite it leads to a modification in particle shape, accompanied by a reduction in adsorption sites, resulting in a decrease in total adsorption. Consequently, contrary to goethite, substitution in ferrihydrite strongly reduces its capacity to trap and transport REE.