The immobilization of lanthanide (III) cations on the Purolite A-400, a cross-linked polymer containing –R₄N⁺ groups, was investigated. This type of polymers theoretically cannot retain metal cations from solutions. We assume that cations sorption on this polymer takes place due to the formation of jarosite mineral–type compounds. By increasing the temperature in the range of 0–60 °C, the cations sorption increases slightly but in the pH range of 2–6, it increases more considerable in the following order: La³⁺˂ Nd³⁺ ˂ Eu³⁺ ≈ Er³⁺. The sorption at 21 and 50 °C is adequately described by the pseudo-first-order (PFO) kinetic model, and the limiting step of the rate which decreases in order Er³⁺ ˃ Eu³⁺ ˃ Nd³⁺ ˃ La³⁺ is internal diffusion.