Gypsum calcination in superheated steam is investigated. It is shown that superheated steam positively influences the formation of hemihydrate and avoids formation of anhydrite. By measuring the specific surface area the products are shown mainly to consist of alpha-hemihydrate. By recovering the heat of vaporization a specific energy use of 379 kJ/kg dihydrate can be obtained. This is substantially lower than the energy used both in existing alpha-hemihydrate and beta-hemihydrate processes. A process flow-sheet is proposed based on the presented results.