Two corrosivity classification systems are proposed for the underground environment with relation to the corrosion of rock bolts. The corrosivity of the underground environment is judged on the basis of a number of corrosion-related parameters. The first classification system is for wet rock conditions. The corrosion-related parameters used in this system are the pH value of the water, the dissolved oxygen and the resistivity, as well as the ambient temperature, rock mass quality and precipitation of solid substances on the rock bolts. The second classification system is for dry rock conditions. The corrosion-related parameters used are the deposition rate of sulphur-and-nitrogen oxides and chloride and the relative humidity of the atmosphere, as well as the ambient temperature.