The measurement of harmonic and interharmonic voltages up to 2.5 kHz is an inherent part of the Power Quality assessment in electrical networks. In the past the monitoring of harmonic voltages has been of minor importance for many transmission system operators (TSO). As the number of power electronics connected to the transmission grid (e.g. self-commutated HVDC stations or converters in large wind power plants) increases continuously, the concern of TSOs regarding the harmonic levels grows as well. This is also confirmed by the recent significant increase of installed Power Quality monitors in transmission grids [1]. In many cases the traditional voltage instrument transformers (VT), which are only designed to have a high accuracy at rated frequency, are used for voltage harmonic measurements. This questions the accuracy of the harmonic measurements and the reliability of any compliance assessment (e.g. against existing planning levels). The paper illustrates the challenges of harmonic voltage measurements from the viewpoint of a TSO. After a quick description of the initial situation the paper presents and discusses the measurements of the frequency response characteristic of different VTs and their impact on the accuracy of voltage harmonic measurements.