This study was carried out to estimate the reservoir temperatures for three Austrian and three Polish geothermal systems, for which these values had never been estimated before. Estimation was conducted on the basis of available results of physicochemical analyses of geothermal waters, compiled by GeoERA project and published as HOVER map viewer. Reservoir temperatures were estimated using classic chemical geothermometers and the GeoT Application, however reliable values were obtained only with the second method. The estimated temperature values can be used for further analysis and research in the recreation, balneology and geothermal industry. The analysis of the chemical components concluded that the lithology, which determines the mineral composition of the aquifer, is a key factor in the estimation of reservoir temperatures of the selected geothermal systems.