The interaction between impurities of two solute elements in metal hosts leads to a non-random distribution of impurity atoms among the lattice sites. Experimental studies of this distribution provide quantitative information on the energy involved in the formation of an individual impurity-impurity pair. It is shown how this energy can be extracted from PAC and Mossbauer spectra. A review of the experimental results for several metallic matrices is given. The binding energy of the impurity-impurity pairs is discussed in terms of a correlation with the heat of formation for corresponding alloys.