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Different kinds of doping in ZnS crystals lead to characteristic ion charge conversions of iron and chromium ions. This is shown by the esr of Fe3+and Cr+ions and the behavior of the esr lines with respect to illumination with uv. Doping with CuS gives Fe3+ions; ZnS doped with Zinc or halogen shows Cr+resonance after uv irradiation. The ion charge conversions are interpreted by changes in the FERMI level. Iron in ZnS crystals can be converted completely into Fe3+through tempering for 24 hours at 800°C with CuS in evacuated quartz ampullae if the iron contents is not greater than about 10-3% by weight. Fe3+can then be determined quantitatively with electron spin resonance for concentrations between 10-3and about 106% by weight. Tempering in sulphur vapor at 1050°C produces from the originally cubic and submicroscopically twinned crystals such that are hexagonal in the extent of about 60%.