The geochemical facies of Hungarian thermal waters were the object of this study. Samples were separated into groups by relative ages (δ 18 O values). Mature and immature subgroups were formed on the basis of dissolved (semi)volatile organic compounds. The oldest (connate) waters form one group with a small number of samples. The subgroups containing different small molecular-sized soluble aromatics differ sharply in their chemical features (sodium, hydrogen carbonate, iodine, ammonium etc. content). The origin of the organic matter may differ in the subgroups as inferred by their different δ 18 O values and ten times greater halogen contents. Our results show that the decomposition of organic matter produces small molecular-sized aromatic compounds and also influences the amounts of inorganic components in thermal waters, through the increase of feldspar hydrolysis and carbonate dissolution. Key words: hydrogeochemistry, thermal waters, δ 18 O, organic matter, aromatic compounds Introduction Thermal waters in the Pannonian Basin may contain high amounts of dissolved organic material. Humic substances and aromatic content of some tens of Hungarian thermal waters have been described qualitatively by Kárpáti et al. (1995, 1996a,b, 1999). Addresses: J. Fekete, Cs. Sajgó: H-1112 Budapest, Budaörsi út 45, Hungary, e-mail: feketej@geochem.hu, sajgo@geochem.hu I. Horváth: H-1143 Budapest, Stefánia út 14, Hungary, e-mail: horvathi@mafi.hu Z. Kárpáti: H-1032 Budapest, Kiscelli u. 18, Hungary, e-mail: karpati-zoltan@t-online.hu I. Vetõ: H-1026 Budapest, Balogh Ádám u. 18/c, Hungary, e-mail: vetoie3840@gmail.com M. Hetényi: H-6722 Szeged, Egyetem u. 2–6, Hungary, e-mail: hetenyi@geo-u-szeged.hu Received: April 2, 2010; accepted: August 8, 2010 1788-2281/$ 20.00 © 2009 Akadémiai Kiadó, Budapest Central European Geology, Vol. 52/3–4, pp. 269–285 (2009) DOI: 10.1556/CEuGeol.52.2009.3–4.4 Interaction and variation of isotopic age, temperature, organic and inorganic solutes in Hungarian thermal waters József Fekete, Csanád Sajgó István Horváth Institute for Geochemical Research, Geological Institute of Hungary, Budapest Hungarian Academy of Sciences, Budapest Zoltán Kárpáti István Vetõ Budapest Budapest Magdolna Hetényi University of Szeged, Szeged