Springwater samples were collected at ¿guas da Prata, an important spa in S¿o Paulo State, Brazil. The 238U and 234U isotopes were evaluated for Platina, Paiol, Villela, Prata-Radioativa, Prata-Nova, Boi, Vit¿ria, and Prata-Antiga springs. An inverse logarithmic correlation was found between 234U/238U activity ratio and 22Rn content of the studied waters, which was related to different solution mechanisms of 222Rn and 234U nuclides at the rock-water interface. A linear correlation also exists between the 234U/238U activity ratios of the studied waters and total dissolved solids, ionic strength, CO2 partial pressure, and index of base exchange, which indicates that the enhancement of 234U in solution increases with the degree of weathering in the aquifers. A high 234U/ 238U activity ratio was found to be directly related to a higher value of dissolution rate. ¿ American Geophysical Union 1993