The Gabal Marwa area is located in the southeastern part of Sinai,Egypt.It comprises gneisses and migmatites,granodiorites and monzogranites.Field,petrographic,mineralogic and chemical investigations indicated that the Gabal Marwa granites are classified as granodiorites and monzogranites.The monzogranites constitute the most predominant rock unit exposed in the study area.They have been subjected to hydrothermal alterations,especially along the shear zones.Sericitization,desilicification,nametasomatism and development of spotty or dendritic manganese oxides are the most pronounced alteration features.These alterations resulted in the increase of TiO2,Al2O3,FeOt,MnO,CaO,MgO,Na2O,K2O and Cr,Zr,Rb,Y and Sr and the decrease of SiO2,P2O5,Ni,Zn,Ba and Nb.Radiometric studies indicated that the altered granites belong to high thorium,high uranium granites.The U,Th,U and Th,Th/U,Th and U-K variation diagrams suggested that magmatic processes controlled the distribution of these elements but the effect of hydrothermal alteration processes were clearly distinct.The Scanning Electron Microscope and X-ray Diffraction analyses indicated that the most important radioactive minerals include uranothorite,thorite,zircon,monazite and samarskite.Cinnabar and Mn minerals were also found.From the U,Th,Ra and K activity concentrations obtained for all the studied granitic samples,radium equivalent activity(Raeq),external hazard index(Hex),and internal hazard index(Hin),were calculated to assess the radiation hazard to human beings living in dwellings made of the studied granites.Altered monzogranites have radioactivity above the proposed acceptable level and,therefore,caution must be taken when these granites are used as building materials. The Gabal Marwa area is located in the southeastern part of Sinai, Egypt. It includes gneisses and migmatites, granodiorites and monzogranites. Field, petrographic, mineralogic and chemical investigations that the Gabal Marwa granites are classified as granodiorites and monzogranites. Monzogranites constitute the Most predominant rock unit exposed in the study area. They have been subjected to hydrothermal alterations, especially along the shear zones.Sericitization, desilicification, nametasomatism and development of spotty or dendritic manganese oxides are the most pronounced alteration features. these alterations resulted in the increase of TiO2, Al2O3, FeOt, MnO, CaO, MgO, Na2O, K2O and Cr, Zr, Rb, Y and Sr and the decrease of SiO2, P2O5, Ni, Zn, Ba and Nb. Radiometric studies indicate that the altered granites belong to high thorium, high uranium granites.The U, Th, U and Th, Th / U, Th and UK variation diagrams suggest that magmatic processes controlled the distribution of these elements but th e effect of hydrothermal alteration processes were clearly distinct. The Scanning Electron Microscope and X-ray Diffraction analyzes indicated that the most important radioactive minerals include uranothorite, thorite, zircon, monazite and samarskite. Cinnabar and Mn minerals were also found. Fro the U, Th, Ra and K activity profiles obtained for all the studied granitic samples, radium equivalent activity (Raeq), external hazard index (Hex), and internal hazard index (Hin), were calculated to assess the radiation hazard to human beings living in dwellings made of the studied granites.Altered monzogranites have radioactivity above the proposed acceptable level and, therefore, caution must be taken when these granites are used as building materials.