Abstract: Dolomites (Ballikay a formation) exposed in the Triassic succession of Kocaeli Peninsula are 15-25 m. thick The dolomiteswere formed on a transition area between slope and basin. They consist of two different types as matrix and cementationdolomites. Stable isotope values obtained from those are 8I8O = -5.18 - -6.10 %o; SUC= 1.49 - 2.81 %o . The isotopic datasuggest that an environmental change took place during dolomitization, which ranged from fresh-seawater mixing zone toshallow burial realm. On the other hand, dolomitization temperature calculated as 36-41 C indicates that the dolomites hadnot suffered from deep burial diagenesis.