In this work, radon concentrations were measured in four buildings in the Diriyah campus of King Saud University, Riyadh, Saudi Arabia. The closed chamber technique was used employing 2x2 cm 2 sheets of CR-39 detectors. The detectors were kept for periods of about 3-4 months. A total of 270 measurements were taken, 30 of which were found to exceed the EPA limit of 150 Bqm -3 . These high ... [Show full abstract] concentration values have a range of 156 to 235 Bqm -3 , 156 to 254 Bqm -3 , 150 to 292 Bqm -3 and 152 to 256 Bqm -3 in buildings one, two, three and four respectively. The average radon concentration per room in each building is found to be 78 ± 5, 82 ± 5, 50 ± 5 and 58 ± 5 Bqm -3 in buildings one, two, three and four respectively. The effects of factors such as height, ventilation, soil and building materials in this study were investigated. On the basis of this study, no significant radon concentration dependence on height was observed and the influence of ventilation is not significantly observed as expected. Therefore, the ventilation is not the only contributory factor in radon variation. It is concluded that soil and building materials are dominant sources of the measured high values in this study.