The radon exhalation from samples of concrete, alum shale and alum shale bearing soil was studied as a function of the moisture content of the samples. A few simple experiments showed that, up to a certain moisture content, the radon exhalation increased with increasing moisture content. In some of the samples, the radon exhalation increased by a factor of almost 20. When the moisture content was increased further, the radon exhalation decreased to levels comparable to the exhalation from dry samples. The radon exhalation as a function of temperature was studied by experiments performed at 5 oC, 22 oC and 50 oC. The radon exhalation increased with increasing temperature. This effect was, however, less pronounced than the effect of the moisture.