The Eskisehir magnesite deposits (Suleymaniye, Margi, and Tutluca) are located in the western part of the Izmir-Ankara Suture Zone, northwestern Turkey. These vein and stockwork type magnesite deposits, which occur along major and minor fault systems, are hosted by Alpine-type ultramafic rocks. The purpose of this study was to understand the origin of the hydrothermal waters responsible and the source of carbon dioxide, and to compare these deposits with similar magnesite occurrences in Turkey and elsewhere. Petrographic and XRD analyses indicate that magnesite was the major carbonate mineral formed. Deposits are predominantly micritic and locally microsparitic, but some also contain secondary calcite and dolomite. The delta C-13 (V-PDB) values of the Suleymaniye magnesites (-2.7% to -7.7%), the Margi magnesites (-7.6% to -11.2%), and the Tutluca magnesites (-8.7% to -10.4%) indicate that sources of carbon may include atmospheric carbon, dissolved inorganic carbon, freshwater carbonate, and mantle derived CO2. The delta O-18 (SMOW) compositions of the magnesite range from 27.4% to 30.8% and show that the oxygen was derived from marine limestone and metamorphic rocks. The Suleymaniye magnesites have heavier carbon isotopic values than the others because of the greater contribution of mantle sourced CO2, while oxygen isotopic values were similar to those of other altered ultramafic related magnesites in Europe (Former Yugoslavia, Greece). The Margi and the Tutluca magnesite deposits have carbon and oxygen isotopic values similar to those of other ultramafic-related magnesite deposits. Based on isotopic data, we argue that the magnesite deposits in the Eskisehir area formed in a near surface environment at low pressure and temperature. The estimated temperature, using average delta O-18 values, suggests that magnesite was precipitated from water at 37 degrees C.