A miniature high precision conductivity and temperature (CT) sensor system has been developed for ocean salinity monitoring. The CT sensor is manufactured using micro fabrication technology. A novel seven-electrode conductivity cell has been developed which has no field leakage. This is combined with a platinum resistor temperature bridge to produce an integrated CT sensor. A generic impedance measurement circuit has been developed, with three-parameter sine fitting algorithm. It has a 1 month battery life at 10 s sampling interval. Calibration results show that the initial CT accuracies are ±0.03 mS/cm and ±0.01°C, respectively. Testing of the CT sensor has been performed in the north Atlantic and revealed drift in sensor readings after five weeks of operation.