This work presents results of the measurements of thoron (Rn-220) and radon (Rn-222) activity concentration in 23 mineral waters sources occurring at the cities of Aguas da Prata, Pocos de Caldas, Aguas de Lindoia, and Aguas de Sao Pedro that are spas located at Minas Gerais and Sao Paulo States, Brazil. Physico-chemical parameters and dissolved gases have been also reported to understand the circulation environment of the waters, which indicated that it varies from transitional to reducing conditions. The thoron and radon activity concentrations ranged from 0.01 to 0.93 Bq/L and from 0.06 to 104 Bq/L, respectively. According to the Brazilian Code for Mineral Waters, these activities indicated that none of them are thoriferous, whereas two are weakly radioactive. The highest Rn-220 and Rn-222 levels were found in sources located in Pocos de Caldas and Aguas da Prata cities, within the Pocos de Caldas alkaline complex. This massif is a well-known suite of alkaline volcanic and plutonic rocks that possesses an accumulation of U, Th, other natural radionuclides, and rare-earth elements. In Villela spring, Aguas da Prata city, Sao Paulo State, the radon activity concentration exceeded the maximum allowed value of 100 Bq/L in drinking water as proposed by WHO in 2009. Although this spring is not utilized in a regular public water-supply system, there are taps available in a park allowing the water consumption by population. However, it has not been observed any health impact on local people that would need action taken by the Government.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)