The use of water activity probes immersed in the vegetable oil dielectric of a transformer requires a fresh look at the equations, and assumptions, behind the conversion of water activity to concentration of dissolved water. This is important because water negatively affects the effectiveness of insulation. A large volume of work has been performed on mineral oil based dielectrics to determine mathematical relationships. However, there is a lack of data on whether these relationships hold for vegetable oil. One example is the appropriateness of using the Henry's law coefficient of proportionality to calculate the concentration of water dissolved in the fluid from its water activity. This paper presents research investigating the water activity-concentration relationship for one vegetable oil dielectric. It was found that, at room temperature under the prescribed conditions, using Henry's law is appropriate.