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Abstract: The study area, located at the contact zone between the Harghita Mountains and the Transylvanian Basin is well known for the abundance and diversity of mineral water springs. These mineral waters are used by the local people for drinking and also as Spas. This paper focuses on the mineral waters of some traditional Spas from this area, Odorheiu Secuiesc (Seiche Spa, Kápolnás Spa or Solymossy Spa), Upper Vârghiş Valley (Selters Spa and Nádasszék Spa) and Chirui Spa. The aim of the present work is to characterize the mineral waters based on the quantification of chemical content and also to establish correlations between the geological structure and the water''s chemical composition. The geological setting and tectonics have a large influence on the chemical composition of mineral waters. Two main water types can be described, mineral waters of Na+ – Cl- type, present at Odorheiu Secuiesc, Kápolnás Spa and of HCO3- type with no dominant cation at Chirui Spa. The mineral waters located on a fault at Odorhei Seiche Spa have high CO2 values. Possible mixing zones might be identified between the two types which are present in the Upper Vârghiş Valley where mineral waters of Na+ – HCO3-– Cl- type are present except one source, which is of Na+ – Cl- type. The mixing is reflected not only by the chemical composition, but also by the quantity of dissolved CO2. The present paper deals with all the existing mineral water springs in the area providing information in a high resolution about the local hydromineral system. [Copyright &y& Elsevier]