Models of an association solution, i.e., a regular solution with two sublattices, a quasi-chemical sublattice and a coordination-cluster sublattice, are used to describe the interaction of the components in diluted liquid-metal solutions. The influence of hydrogen upon the solubility of the metals in the liquid lithium was studied in our work, making use of the coordination-cluster model of a solution [2]. This model was selected because in it there is good agreement between the calculations and the experimental data for the Li-Pb-H system [3] and also because with it a comparison of the interaction parameters with the experimentally obtained data for ternary hydrogen-containing solutions on the basis of iron has been made [4]. It is assumed in the model that the atoms of a nonmetallic element C (hydrogen) in a melt of two metals A and B (A being lithium and B, a dissolved metal) occupy interstitial positions with the coordination number z. Each C atom in the melt has 1B atoms and (z - i) A atoms as the nearest neighbors, where i can vary between 0 and z. There exist (z + i) varieties of these configurations in the melt, which are termed clusters and denoted by C(Az_ i - Bi). The thermodynamic parameters of the A--B-C system must depend upon the distribution of the C atoms over the clusters, the distribution is given by I) the interaction of the atoms of the nonmetal with the metal atoms; 2) the interaction between the metal atoms which are in a cluster; and 3) the interaction of the metal atoms in the cluster with the neighboring atoms of the solvent metal. An expression for the parameter of the interaction of the element C with the metal B atoms in the solution was obtained with the methods of statistical mechanics [2]: e~(OInyc(A+B+C)/ON#)NB.tI= --z{Y~(YC~A/Yc(B)) 1/zexp[-(z-i) M(2Rr)]-l}' where ?C(A+B+C) denotes the activity coefficient of the element C in the ternary system; N B denotes the molar fraction of the element B in the solution; ~B denotes the activity coefficient of metal B in the binary A-B system for N B ~ 0; 7C(B) and ~C(B) denote the activity coefficient of element C in liquid solutions based on metals A and B, respectively; h denotes an energy parameter accounting for secondary effects related to the difference in the charge transfer from C atoms to the metal atoms in the clusters of each type; and t denotes a gee