We report field observations and C and O isotopic compositions of two magnesite deposits within the Lower Cretaceous Dagkuplu ophiolite complex, Eskisehir-Kutahya region, Turkey, to help interpret their genetic significance. The Nemli and Margi deposits are large microcrystalline stockwork-type magnesite systems within the upper part of highly fractured and imbricated ultramafic complexes containing fresh dunite, harzburgite and serpentinite. Magnesite in ultramafic rocks occurs either as stockwork veins or as irregular masses. Centimeter-scale chalcedony bands are observed within magnesite veins. Some serpentinite fragments and platelets are rimmed by fibrous magnesite, indicating significant alteration of the host rock along the WNW-ESE trending Eskisehir fault zone, which is subparallel to the northern edge of Neogene lacustrine sediments. The magnesite deposits are related to tectonic activity along this fracture zone and Middle Miocene calc-alkaline volcanism.