MANY geochemical processes, such as crystallization of silicate magmas or planetary differentiation, require a knowledge of the way in which elements become partitioned between coexisting crystal and liquid phases(1,2). But quantitative prediction of crystal/melt partition coefficients from thermodynamic principles has not previously been possible. By studying the partitioning of 15 elements between silicate minerals and their coexisting melts, we show here that the partitioning behaviour of any series of isovalent cations can be rationalized in terms of a simple model in which the size and elasticity of the crystal lattice sties play a critical role. We find that elasticity varies linearly with the formal charge of the cation. This model allows us to predict element partitioning behaviour solely from the physical characteristics of the cation sites in the crystal.