Selenium has a radiotoxic potential as 79Se radionuclide if discharged from a high-level nuclear waste repository into the biosphere. We performed three different syntheses of selenium doted pyrite and examined the amount of incorporated Se as well as the structural bonding. The incorporated amount of Se is very high (>98%). The structural substitution of Se by S occurs only for high supersaturated solutions. Under anoxic and acidic conditions, the incorporation of Se(0) is more likely.