Maturity functions are not valid above a thirty-percent degree of hydration. The effects of moisture at an early age on the hydration process are still unclear. More information is needed about the relationship between compressive strength and the degree of hydration as a function of moisture state - or rather RH. Such information will help to improve the maturity function with regard to hydration._x000D_ _x000D_ The purpose of this paper is to evaluate the degree of hydration and the maturity of a cement-based material as functions of RH. A correlation is suggested between maturity and hydration above a certain degree. When a cement-based material is being cured in the air, hydration ceases as soon as the RH drops below a certain level, which appears to be about 80% according to Powers /9/._x000D_ _x000D_ Hitherto it has been extremely difficult to establish the degree of hydration at limited moisture levels for various cement-based materials. The RH of the material might drop critically at an early maturity age . There is, however, a serious lack of published data on the effects of microstructural development under partially saturated conditions. My purpose is to provide data on ordinary Portland cement pastes and mortars with w/c-ratios of 0.35, 0.5 and 0.75. With the help of a special technique which has been developed at CBI, such materials were transformed to various RH after certain initial curing times. The influence of additives is not considered in this study._x000D_ _x000D_ After a certain period of hydration at the new RH, changes in hydration and microstructure were analysed for non-evaporable water and by quantitative X-ray diffraction._x000D_ _x000D_ This study shows the sometimes striking influence of drying or partially saturated conditions on the reduction of the rate of hydration, the final degree of hydration and the microstructure.