The falaj is a man-made stream which intercepts groundwater at the footslopes of mountains and brings it to the surface at a lower level for irrigation purposes. The objective of this study is to discuss the effect of climate, geology and hydrological conditions on the chemistry and quality of water in the U.A.E. falajes. Between 1978 and 1995, total falaj discharges ranged from 9.0 to 31.2 × 106 m3/yr, which represents 2.8 to 9.7% of the total water use in the country. The location, aquifer storage, seepage loss from falaj channels, and rainfall, are the main factors affecting falaj discharges in the U.A.E. All U.A.E. falajes are confined to the northern Oman mountains and the surrounding gravel plains. The Electrical Conductance (EC) of falaj waters are generally low to medium, varying between 450 micro-Seimens per centimeters (μS/cm) in Falaj Asimah (Al Fujairah) and 10 940 μS/cm in Falaj Ain Sukhnah (Al Ain). Iso-EC map shows that the EC of falaj waters is minimal near the water divide of the mountains and increases to the east and west. In open-channel falajes (Al Gheli type), the EC increases with increasing falaj length, but in the tunnel-type falajes (Al Daudi type), EC is generally low irrespective to the falaj length. Falaj waters have high concentrations of Mg2+ because they drain magnesium-rich mantle sequence rocks of the lower Semail nappe. The Mg/(Ca+Mg) ratio is >0.5 in 27 falajes out of 33, indicating the dissolution of Mg-rich rocks which are the main constituent of the ultramafic rocks of the Semail ophiolites. These conditions also favor the precipitation of calcite (CaCO3) and possibly huntite (Mg3Ca(CO3)4) which can be altered later to magnesite (MgCO3). According to EC and Sodium Adsorption Ratio (SAR), water of the Gheli falajes in ophiolitic rocks are good for irrigation, whereas the water in the Daudi and Hadouri falajes draining limestone rocks are fair to poor for irrigation purposes.