Abstract:  A well known problem in the grinding procedure, in active gypsum open pits, is the difference of mechanical properties between gypsum and anhydrite. Specifically, the lower stiffness of gypsum compared to that of anhydrite, causes different energy requirements in cement production line. For these reasons it is necessary to be aware of the spatial distribution of these rocks in an active open pit. In this paper, the 2D resistivity method, in combination with detailed geological mapping, is tested as tool for the mapping of gypsum/anhydrite transition zones. A series of geophysical profiles were scanned in selected locations, following the geological mapping of the mine site, in order to locate gypsum/anhydrite transitions. In addition, the contacts of the deposit with the underlying schist formation were delineated. The results from the geophysical survey will be compared with the data from the geological mapping and the drills performed in 1982 during the exploration of the active gypsum open pit of INTERBETON S.A. in Altsi (Sitia region, Εastern Crete). This knowledge, because of its strict connection with the calculation of the reserves (geological and recoverable), contributes to the optimal production planning of the open pit.