The distribution coefficients D of yttrium and the lanthanides between solid and liquid natural aluminosilicates can be described in terms of an extension of Schuetze's [1] incremental method for calculating oxygen isotope effects in silicate systems: log D = k + S · ΔI, where, ΔI the difference of the18O indices of the liquid and the solid phase and k and S coefficients to be determined empirically. The coefficient k accounts for the effect of the aggregational state, particularly the water content, S accounts for the effect of the degree of linking of the (Si, Al)O4tetrahedrons in the phases being in equilibrium on the distribution coefficients. The dependence of D, k and S on the atomic number can be understood in terms of the radii of the hydrated and not hydrated lanthanide ions. The constraints on the structural properties of magmas are discussed.