In the ultrahigh vacuum (UHV) contactless capacitance-voltage (C- V) technique, a narrow UHV-gap(300-400 nm) is maintained between the electrode and the sample surface by a capacitance feedback and a piezo-electric control mechanism. This thin gap acts as an additional insulator and allows standard MIS assessment even for a free and ultrathin insulator covered semiconductor surfaces. Using this technique, nearsurface macroscopic electronic properties such as the conduction type, doping density, surface/interface state density distribution and Fermi level pinning position can be determined quantitatively. Here, the electronic properties of the hydrogen-terminated silicon surfaces and ultrathin insulator covered silicon surfaces were characterized. X-ray photoelectron spectroscopy (XPS) measurements were also done. Hydrogen terminated silicon surfaces possess fast surface states and slow surface states. By the sufiiciently long NH_4F treatment followed by UHV anneal at 200℃, a UHV-compatible completely passivated surface could be obtained. The most likely origin of fast surface state is silicon-dangling bond, and that of slow surface states is excess hydrogen. On the other hand, relatively well-behaved ultrathin insulator/silicon interface was obtained after electron cyclotron resonance (ECR) N_20-plasma oxynitridation process at 400℃. Formation of a phase-separated Si_3N_4/Si0_2 structure is important to realize a well-passivated interface. The electronic and chemical properties of the interfaces showed strong correlations.