Abstract <p>Sorption of lanthanum(III) and cerium(III) ions on the Na and H forms of clinoptilolite was studied in wide ranges of time (60 min–48 h) and concentrations (0.01–0.05 N) of La(NO₃)₃⋅6H₂O and CeCl₃⋅7H₂O solutions. The ion-exchange equilibrium in the system (S : L) on the Na and H forms of clinoptilolite is achieved in 48 and 24 h, respectively. The kinetic (in statics) parameters of ion exchange of Ce(III) and La(III) on the monoforms of clinoptilolite were calculated. An intra-diffusion mechanism of ion exchange was established. The presented isotherms of ion exchange of La(III) and Ce(III) for the Na form of zeolite are convex, and those of the H form have an inflection, which corresponds to the values of the ion exchange constant calculated using the empirical Nikolsky equation (at <i>K</i> &gt; 1 they are convex; at <i>K</i> &lt; 1, concave). The effect of pH of the medium on the sorption capacity of zeolites in the monoforms relative to the triply charged REE cations, in particular, La(III) and Ce(III), was revealed.</p>