Analysis of the gases dissolved in power transformer oil is recognized as the most useful tool for early detection and diagnosis of incipient faults in transformers. In addition, water contamination deteriorates the performance of the oil as high moisture content increases the risk of corrosion and overheating. This is when the water content reaches the saturation point of the oil and free water is formed. Besides regular gas chromatographic analysis and off-line moisture analysis of the isolation oil of power transformers online monitoring systems gain more and more importance worldwide. By online monitoring of the key fault gases Hydrogen (H2) and Carbon Monoxide (CO) and Moisture in Oil (H2O) an additional potential of cost reductions and safety improvements can be achieved.