Hydraulic fluids underwent significant changes in their composition in these past years, which led to an increase in poor conductivity. Since well-established standards for specifying a hydraulic fluid, such as ISO 11158, make no mention of the electric conductivity, the awareness for potential complications is low. With some applications electrostatic discharges contribute to a shortened service life of the hydraulic fluid and the filter elements. Recent work on cavitation and its different forms suggest an underestimated impact of the relative humidity on fluid characteristics and the tendency for cavitation. In relation to electrostatic charges, the relative humidity of a fluid is seldom considered. The results presented in this paper offer an initial observation of the effect of relative humidity on electrostatic charging. To this end three hydraulic fluids, satisfying the specifications of an HLP 46, are prepared with three different levels of relative humidity at 20°C. Subsequently the nine samples are tested with a small-scale test bench in order to garner a qualitative comparison of the charging tendency.