When beryllium is thermally diffused into silicon, it gives rise to acceptor levels 191 meV and 145 meV above the valence band. Quenching and annealing studies indicate that the 145 meV level is due to a more complex beryllium configuration than the 191 meV level. When lithium is thermally diffused into a beryllium doped silicon sample it produces two new acceptor levels at 106 meV and 81 meV. Quenching and annealing studies indicate that these new levels are due to lithium forming a complex with the defects responsible for the 191 meV and 145 meV beryllium levels, respectively. Electrical measurements imply that the lithium impurity ions are physically close to the beryllium impurity atoms. The ground state of the 106 meV beryllium-lithium level is split into two levels, presumably by internal strains. Tentative models are proposed to explain these results. / Ph. D.