Sulfur vacancies were introduced into pure, Al-diffused, and Cu-diffused ZnS crystals by energetic electron irradiation. By measuring the change in intensity of EPR signal arising from the S vacancies on thermal annealing, the S vacancy in the Cu-diffused crystals was found to form a complex center, the so-called Cu-red center, around 200 degrees C. The crystals irradiated at temperatures lower than - 100 degrees C showed very complex EPR signals besides the ones arising from isolated S and Zn vacancies. These complex signals are proposed to arise from complex defects consisting of Zn vacancies and interstitials.