The grindability of raw materials is a key parameter of mineral processing. It is also very important for some kinds of secondary raw materials, such as the demolition and construction wastes. The Institute of Raw Materials Preparation and Environmental Technology of the University of Miskolc has been dealing with this topic for a long time. The Universal Hardgrove Mill and the Universal Bond Mill had been developed. With these devices the grindability can be measured at so called universal circumstances, namely at high temperature in any (acidic or alkali or neutral) medium. The rheological behavior of the ground slurry or suspension evidently affects grinding because of the well-known viscous dampening effect. However, the grindability of materials as function of the moisture content and rheology has not been studied on-line. Therefore, the grindability of a soft adhesive (limestone) and a hard non-adhesive (quartz) material was examined in the Universal Hardgrove Mill during room temperature dry and wet (in tap water) grinding. The powder flow behavior of the dry ground samples was measured in an FT4 powder rheometer, while the rheological properties of the wet ground suspensions were measured in a rotational rheometer.