In concrete structures exposed to the ambient air at early ages, the moisture content in concrete decreases due to moisture diffusion. In addition, self-desiccation due to hydration of cement causes an additional decrease of moisture content in concrete at early ages, especially for high-strength concrete, In this study, the internal relative humidity in drying concrete specimens was measured at early ages. Furthermore, the variation of relative humidity due to self-desiccation in sealed specimen was measured. The moisture distribution in low-strength concrete with high water/cement ratio was mostly influenced by moisture diffusion due to drying rather than self-desiccation. In high-strength concrete with low water/cement ratio, however, self-desiccation had a considerable influence on moisture distribution. The results obtained from the moisture diffusion theory were in good agreement with experimental results. (C) 2000 Elsevier Science Ltd. All rights reserved.