To experimentally demonstrate the capabilities of integrated finline, a cavity test fixture was constructed and tested. The results were analysed to determine the manner in which the guide wavelength and unloaded Q factor varied. The measured unloaded Q factor of integrated finline was found to be two to three times better than that of microstrip. Owing to the low loss of integrated finline, its freedom from tight tolerances and compatibility with hybrid devices, this new transmission line is well suited to millimetre integrated circuits.