Application of pure, oxygen-free copper as a construction material having excllent corrosion resistance is limited because of the effect of intergranular emnibrittleinent at temperatures above 100–150°C. Dilute copper alloys containing S, P, and Ag inpirities and vacancies are studied theoretically on the basis of total energy calculations. The dissolution energies, volunie misfits, and defect interaction energies are calculated and used to study the microscopic miechanisin behind the effect of these impurities on the eiibrittlenieiit of colpper at interinedia temiperatures. A large linding energy of a sulfur-vacancy defect pair (–0.46 eV) is found. The sulfur-vacancy and sulftir-sulftir interactions in the copper matrix seeni to favor precipitation of copper sulphide Cu2S which is the most probable cause of the einhrittleiieiit. The effect of phosphorus and silver ilmpurities on the einbrittlement of sulfiir-contaminated copper can he related to their competition with sulfuir to attract to vacancies as well as to other lattice defects.