Here we demonstrate the proof-of-principle of a coaxial stub resonator to assess the dielectric properties of fluids. This radio-frequency spectroscopy method is based on coaxial stub technology and comprises quarter wave length open-ended resonators that are filled with a liquid sample as dielectric between inner and outer conductor. Changes in the dielectric properties of the liquid sample result in changes in the electric properties of the resonator, e.g., its resonance frequency and quality factor. In addition to a batch resonator, results obtained with a flow-through resonator indicate that the concept can be further developed into a cost-efficient and low-maintenance sensor for the on-line fingerprinting of the dielectric properties of fluids, such as drinking or waste water. ethanol and glycerol.