We present experimental verification of resonant reflection notch method for quasi-optical mm-wave characterisation of dielectric materials, particularly, those with large dielectric losses. The method utilises an open dielectric Bragg resonator with a “defect-layer” insertion that contains the material under test. The structure is tuned to show narrow reflection notches at certain frequencies which are exceptionally sensitive to the measured dielectric quantities. The method is analogous to the “capillary-in-a-waveguide” technique, though implemented in a simpler (planar) one-dimensional system and in reflection rather than in transmission mode. Here we present experimental implementation and verification of the method as applied to the dielectric characterization of water solutions in the millimetre-wave frequency band of 75-110 GHz.