This study focuses on the geochemical and isotopic properties of geothermal and mineral waters in the Mugla region of SW Turkey. In total, 55 water samples - comprised of 38 geothermal water, 16 mineral water, and 1 fresh water - were collected from inland and coastal waters of Mugla. Among the 26 individual sites in the study area, 16 are geothermal and 10 are mineral water sources. The temperature of the geothermal waters ranges between 18.3 and 39.2 degrees C, and thus they are categorized as "low temperature" waters. Two principal processes determine the hydrogeochemical properties of the waters: water-rock interaction and seawater mixing. The hydrogeochemistry of the inland mineral waters is determined by water-rock interaction due to the long residence time of groundwater along the low-angle discontinuity surface between the core and cover units of Menderes Massif. Conversely, seawater mixing controls the chemical character of the coastal geothermal and mineral waters, which results in Na-Cl type waters. The seawater contribution to coastal waters ranges between 3% to 100%. The stable oxygen (delta O-18) and hydrogen (delta D) isotope compositions also imply seawater mixing in the coastal waters and meteoric origin for the inland waters. High angle dip-slip fault systems related to Late Alpine tectonics are extensively responsible for the inland and coastal geothermal waters in the study area. Geothermometer calculations suggest reservoir temperatures ranging between 30 and 164 degrees C for Mugla geothermal sites. (C) 2016 Elsevier Ltd. All rights reserved.