The Oylat spa is located 80 km southeast of Bursa and 30 km south of Inegol in the Marmara region. With temperature of 40 degrees C and discharge of 45 l/s, the Oylat main spring is the most important hot water spring of the area. Southeast of the spa the Forest Management spring has a temperature of 39.4 degrees C and discharge of 2 l/s. The Goz spring 2 km north of the spa, which is used for therapy of eye disease, and cold waters of the Saadet village springs with an acidic character are the further important water sources of the area. EC values of Main spring and Forest Management hot spring (750-780 mu S/cm) are lower than those of Saadet and Goz spring waters (2,070-1,280 mu S/cm) and ionic abundances are Ca > Na + K > Mg and SO4 > HCO3 > Cl. The Oylat and Sizi springs have low Na and K contents but high Ca and HCO3 concentrations. According to AIH classification, these are Ca-SO4-HCO3 waters. Based on the results of delta O-18, H-2 and H-3 isotope analyses, the thermal waters have a meteoric origin. The meteoric water infiltrates along fractures and faults, gets heated, and then returns to surface through hydrothermal conduits. Oylat waters do not have high reservoir temperatures. They are deep, circulating recharge waters from higher enhanced elevations. delta C-13(DIC) values of the Main spring and Forest Management hot spring are -6.31 and -4.45%, respectively, indicating that delta C-13 is derived from dissolution of limestones. The neutral pH thermal waters are about +18.7% in delta S-34 while the sulfate in the cold waters is about +17% (practically identical to the value for the neutral pH thermal waters). However, the Goz and Saadet springs (acid sulfate waters) have much lower delta S-34 values (similar to+ 4%).