The preliminary evaluation of radon levels registered in a building at Ponta Delgada city, São Miguel Island, comprises soil diffuse degassing characterization and indoor air quality analysis. Radon and carbon dioxide (CO2) concentration as well as soil temperature measurements were performed to characterize the soil diffuse degassing emissions in the study area. Temperature and CO2 values were bellow the background value of São Miguel Island. Soil radon concentration varied between 889 and 24000 Bq/m3. A dilution test was also performed. Radon concentration measurements were performed at 60 cm depth, at the soil surface and at 1 m above the soil surface. The results obtained allow us to realize that there is an efficient dilution of the study gas. Indoor radon concentrations were measured in 74 compartments of the monitored building. The values varied between 22 and 1082 Bq/m3. The results showed that this gas is sometimes in concentrations above the recommended by the regional law and by the World Health Organization. Taking into account the measured values a continuous monitoring test was performed on a selected compartment. The values recorded oscillated between 0 and 2460 Bq/m3, with an average value of 460 Bq/m3. From the tests performed we inferred that the opening of windows was normally enough to promote an efficient decrease in the radon concentration.