Electromagnetic characterization of materials is the key parameter for space applications, imaging, and, the high-speed electronics and telecommunications industries. In fact, "a priori" knowledge of different materials' permittivity and permeability can efficiently reduce the cost-of-production and prototyping/testing process based on full electromagnetic simulation and optimization. This requires the reliable characterization of materials over wide frequency ranges with knowledge of the associated uncertainties. In addition, the use of new millimeter-wave and terahertz technology needs to demonstrate conformity with exposure safety limits for both humans and the environment. VNAs can be used to measure materials properties (e.g. complex permittivity) using classic quasioptical systems in millimeter/submillimeter domain or recently commercialized Materials Characterization Kits. The research here will focus on the relatively new commercially available Material Characterization Kits (MCKs) provided by SWISSto12 in 75-100GHz range. These systems follow a "guided free-space" approach using corrugated mode converters to provide a quasi-TEM mode at the location of the sample-under-test. The mode-convertors are very compact and measurements are easy to perform but results are currently lacking traceability. Various calibration and data-extraction methods will be studied to provide reliable measurement results with the associated uncertainties.