The safety of gasholders relies on the reliability of the sealing concept, which usually applies a liquid seal. In the case of a viscosity increase of the sealing medium, the risk of a gas breakthrough multiplies, which must be prevented at all costs. Monitoring of sealing fluid samples from field usage in combination with laboratory tests of model mixtures were used to identify safety influencings variables. Water has proven to be the main factor responsible for most adverse property changes. It acts in both ways, directly altering the liquid’s properties and indirectly promoting corrosion. Hence, water separation is one of the key parameters of the liquids. An adapted method for the determination of water separability was established and the findings from the field monitoring were used to set up a test matrix for further investigations. The findings of this point out the key influencing factors determining the usability of sealing fluids and are applied for improving the liquids by targeted testing methods.