The paper describes an investigation into the dehydration behaviour of a natural and a phosphogypsum under isothermal conditions. The purpose of the work was to assess whether there is a difference in the dehydration behaviour and what effects it could possibly have in practical applications in the cement industry. It was found that there is a significant difference in the amounts of moisture loss from the two materials under the same temperature conditions as well as their respective surface areas after dehydration. The results of the investigation indicate that great care should be exercised under production conditions when changing from the use of a natural gypsum as set retarder for cement to using a phosphogypsum.