A new parallel wire method has been devised for measuring the dielectric constants of fluids and solids at ultra‐high frequencies. A relatively small sample in the form of a thin slab is required. The mathematical relation upon which the experimental determination depends involves only the wave‐length at which the measurements are made, the thickness of the sample, and a bridge shift along the parallel wires measured in air. This relation is derived, the experimental technique is outlined, and sample results obtained with water and alcohol are given.