Serpentinites in the Eastern Desert of Egypt were mapped from Landsat thematic mapper (TM) data by using procedures that take advantage of the distinctive spectral reflectance of these rocks caused by the abundance of antigorite, lizardite, clinochrysotile, and magnetite. The method employs a threshold classifier based on three reflectance ratios: (1) band 5/7 for estimating the abundance of hydroxyl-bearing phases, (2) band 5/1 for magnetite content, and (3) the calculated value of reflectance for band 4, based on a linear interpolation between bands 3 and 5, divided by the observed band-4 reflectance. The third ratio was used to identify rocks rich in iron-bearing aluminosilicates and thereby to distinguish mafic rocks containing substantial amounts of magnetite and hydroxyl-bearing phases from serpentinites. The method was first successfully tested over the Meatiq dome and Wadi Ghadir areas, where serpentinites and ophiolitic melanges dominated by serpentinites have been mapped during the course of field work. A TM-based map was then generated; the map covered about 60 000 km2 in the Eastern Desert. Results demonstrate that TM data can be used with reliability to distinguish serpentinites from surrounding rocks in arid regions and to generate detailed maps over wide regions by using quantitative, reproducible mapping criteria. Possibilities for locating suture zones over the less well known parts of arid continents are clear.