This thesis focuses on measuring techniques and results of resistivity in four commercially available insulating transformer oils: mineral oil, ester oil and two isoparaffin oils. Two measuring techniques, the industrially used diagnostic system for electrical insulation IDA and the Labview implemented Triangular Method, are used for resistivity measurements and the techniques are compared, a correction algorithm to the triangular method is suggested. Dielectric properties of mineral & ester and isoparaffin A&B mixtures are investigated, it is experimentally shown that the transformer oils that show high resistivity also show low loss factor. The effect moisture has on resistivity in mineral and ester oil are shown both in terms of relative humidity and actual water content in parts per million. A previous measurement cell is redesigned, the cell is manufactured in copper and gold. It is found that the material choice of the cell significantly affects the resistivity measurements. The electrical double layer and contact resistance between the oil and cell are investigated as a way to explain the difference in measured resistivity. These experiments are limited to the mineral oil and isoparaffin oil A, it is found that contact resistance is a plausible explanation. The electrical double layer is fairly constant for both oils and the Debye length of the double layer is negligible compared to the total distance between the electrodes of the cell. Lastly, the field of insulating transformer oils and its future is discussed, from data obtained regarding the dielectric properties and environmental aspects of the four transformer oils used in this study. A positive trend which combines the high insulating properties with good biodegradability qualities is found. Suggesting a positive future in the field of insulating transformer oils. The results found in this thesis can be used as a basis for future theses regarding transformer oils used for HVDC applications.