The concentrations of B and Cl in natural waters in Iceland lie in the range 0.001-10 and 1-20,000 ppm, respectively. Lowest concentrations occur in surface and groundwaters in the central highlands. Highest Cl concentrations are found in high-temperature waters on the Reykjanes Peninsula and highest B concentrations in well waters from high-temperature geothermal systems in the axial zones of the active volcanic belts. The B and Cl contents of Icelandic basalts are 0.1-6.6 and 75-750 ppm, respectively. These large variations are considered to result from variable degassing during consolidation as well as concentration variations in the initial magma. Tholeftes are lowest in both elements (av. 1.2 ppm B and 170 ppm CI), transitional basalts intermediate (av. 2.1 ppm B and 240 ppm CI), and alkali-basalts highest (av. 3.8 ppm B and 340 ppm Cl). Cl/B ratios in tholeiites are most often within 25-50, but somewhat lower in alkali basalts. Most geothermal waters contain B and Cl within 0.05-1 and 10-100 ppm, respectively. These rather low values are attributed to the low content of B and Cl in the basaltic rock. The B and Cl distribution in the Icelandic natural waters indicate that these elements act as essentially incompatible. Their sources of supply include (1) the atmosphere (seawater spray and aerosols), (2) the rock with which the water interacts, (3) seawater-groundwater, and (4) magma intrusions. In surface water and nonthermal groundwater the dominant source of supply of Cl is seawater spray and aerosols, but the soil and rock contribute often significantly to the B in these waters. Their Cl/B ratio is similar to that of seawater (1330) or somewhat lower. Boron and Cl concentrations generally increase with water temperature. Warm waters (<50°C) usually possess Cl/B ratios intermediate between those of seawater and basalt. With increasing temperature this ratio decreases and gradually approaches that of the rock. Some high-temperature geothermal waters have as low Cl/B ratios as 1 and B concentrations as high as 10 ppm. The cause is either B degassing of the magmatic intrusive heat source or phase separation in the producing aquifer. Some low-temperature waters (<150°C) in low lying areas by the coast have elevated Cl concentrations due to the presence of a component of seawater-groundwater that entered the bedrock during the deglaciation period when the lowland areas were submerged. Estimation of the marine component of Cl in waters of the low-temperature systems that do not contain a seawater-groundwater component indicate that the recharge is most often local, but not distantly derived from elevated areas inland as has been inferred from their deuterium content.