Integrating NbTiN-based microstrip tuning circuits with traditional Nb superconductor-insulator-superconductor (SIS) junctions enables the low-noise operation regime of SIS mixers to be extended from below 0.7?to?1.15?THz. In particular, mixers incorporating a NbTiN/SiO2/NbTiN microstrip tuning circuit offer low-noise performance below 0.8–0.85?THz, although their sensitivities drop significantly at higher frequencies. Furthermore, a microstrip geometry in which NbTiN is used as the ground plane material only (NbTiN/SiO2/Al) yields significant improvements in the sensitivities of SIS mixers operating up to 1.15?THz, with an upper operating frequency that depends upon the quality of the NbTiN layer, and thus its deposition process. Films deposited at room temperature have Tc = 14.4?K and ?n,20?K ? 60????cm, and offer low-noise performance up to 1?THz, whereas films deposited at 400?°C have Tc = 16?K and ?n,20?K ? 110????cm, and offer low-noise performance up to 1.15?THz. Taken together, these results demonstrate that the high-frequency surface resistance of a NbTiN layer depends upon the film’s structural properties. Most significantly, the drop in performance that is seen at F>1?THz in mixers incorporating NbTiN ground planes deposited at room temperature is attributed to nonhomogeneities in the structural and electrical properties of these films, as is the poor performance of mixers that incorporate NbTiN wiring layers at F>0.85?THz. The development of these NbTiN-based microstrip tuning circuits will enable the production of low-noise SIS mixers for the 0.8–0.96- and 0.96–1.12-THz frequency bands of the Heterodyne Instrument for the Far Infrared on board the European Space Agency’s Herschel Space Observatory.