An estimate of deep groundwater temperature is necessary for the research and utilization of this geothermal resource. Geothermometers are based on the temperature relation of some chemical reactions or the solubility of some minerals. Researchers mostly use silicon-based (quartz, chalcedony, amorphous silica) and cation-based (Na-K, Na-K-Ca, Na-K-Mg, and so forth) geothermometers. Temperatures of some prospectively abundant geothermal water resources in Serbia are estimated using the silicon-based geothermometers. In the absence of hot water resources, temperature of deep thermal groundwater has to be estimated and considered for power generation with the Kalina or the Rankine binary cycle. Best thermal waters (temperatures from 130° to 160°C) for the purpose are located in the spa of Vrnjačka Banja, followed by Kuršumlijska, Sijarinska and Jošanička spas and Bogatić of Mačva. Pumped at the present rate of 200 l/s, the mentioned sources may generate 70 [MWt] , of which some 30 [MWt] the Vranjska Banja alone. Total power (for the five tested resources) is estimated at about 2200 [TJ] per year. [Projekat Ministarstva nauke Republike Srbije, br. 43004]