It is well known that both copper and silver, when alloyed with many other elements, are able to form primary solid solutions of the substitutional type, in which the solute atoms replace those of the solvent upon its lattice so that the crystal structure of the parent metal is retained. The solubility limits of many of these solid solutions have been determined experimentally, but little progress has previously been made in discovering general principles or a quantitative theory. In the present paper we confine our attention to the alloys of copper and silver with the elements of the B subgroups, including those of the two first short periods.