Synopsis The effects of moisture upon concrete properties and performance are briefly reviewed. A system for nondestructively measuring the development of relative humidity and water content profiles in a uniaxially drying concrete cube is described. Relative humidities are measured with a purpose-built, miniature probe. Preliminary results show that the surface layers of concrete dry rapidly enough to impair curing of the cover zone that protects reinforcing steel. At depths greater than 20 mm drying was relatively slow. Water content results indicate that the surface layers are more porous than the underlying concrete. The measured effects of depth, OPC replacement and water/binder ratio upon the variations of relative humidity with time can be described with a simple empirical equation that incorporates a hyperbolic time function. The relative humidity dropped faster the lower the water/binder ratio and the higher the level of OPC replacement with pulverized-fuel ash or ground granulated blastfurnace slag.