The present work investigated the radioactivity level of the granitoid plutons and its effect on the groundwaters in the southeast part of Eskisehir. Fourteen granitic samples from the Kaymaz and Sivrihisar plutons and 11 groundwater samples from the near vicinity of the pluton were analyzed. The activity concentrations measured for U-238 and Th-222 ranged from 43.59 +/- 2 to 651.80 +/- 24 Bq/kg, and 51.16 +/- 3 to 351.94 +/- 13 Bq/kg, respectively. The activity concentrations obtained for 4 K varied from 418.50 +/- 17 to 1618.03 +/- 66 Bq/kg. The absorbed dose rates in air outdoors ranged from 87.14 to 531.81 nGy/h. All the results obtained from the Kaymaz pluton are higher than those from the Sivrihisar. The U (ave. 16.6 ppm) and Th (ave. 49.9 ppm) values of the Kaymaz pluton are higher than the average concentrations of the magmatic rocks of granitic composition. These results are consistent with high dose rates of the pluton. The gross-a activities in the groundwater samples ranged from 0.009 to 1.64 Bq/1 and the gross-beta activities from 0.006 to 0.89 Bq/1. The highest gross-a value was found in the sample taken from near the Kaymaz pluton. The concentrations of Rn-222 varied from 0.060 to 0.557 Bq/1. (c) 2005 Elsevier Ltd. All rights reserved.