Geothermal waters originating from three aquifers in northwestern Romania have been analyzed for several physico-chemical parameters namely, pH, temperature, electrical conductivity, total dissolved solids, hardness, redox potential, and chemical oxygen demand. In addition, major dissolved ions and organic matter content and radium and radon occurrence have been analyzed. The three aquifers are the fissured Middle Triassic limestone/dolomite aquifer from Oradea, the Lower Cretaceous limestone aquifer from 1Mai-Felix Spa, and the Lower Pontian (Late Miocene) granular (sand) aquifer from Sacuieni. The first two aquifers belong to the Inner Dacides and the last to the Pannonian Basin. We have examined the relationships between the occurrence of radium and radon in the thermal waters and the hydrochemical and geothermal features of the aquifers, which may impart a secondary control on the activity of these radionuclides. According to their chemistry, the thermal waters were classified as sulfate-bicarbonate-calcium-magnesium (Oradea), bicarbonate-sulfate-calcium-magnesium (Felix-1Mai Spa), and bicarbonate-sodium-chloride (Sacuieni). The activities of radon and radium were higher in Sacuieni (up to 22.88 and 1.40 Bq/L) and Oradea (up to 34.82 and 1.82 Bq/L) than in Felix-1Mai Spa (up to 16.76 and 0.19 Bq/L). The data shows that the thermal processes may control the migration and distribution of radionuclides, and that the mobility of radionuclides may be influenced by several physico-chemical parameters, such as pH, total dissolved solids, and redox potential, or by some chemical processes, such as adsorption on humic acids or partitions into the organic (hydrocarbons) phase. The data illustrate no significant differences between the parameters analyzed from February to July 2009, proving that the production regime of the aquifers has a minor impact on their chemical and physical parameters. (C) 2011 Elsevier Ltd. All rights reserved.