Two-dimensional layered semiconductors such as molybdenum disulfide (MoS 2) at the quantum limit are promising material for nanoelectronics and optoelectronics applications. Understanding the interface properties between the atomically thin MoS 2 channel and gate dielectric is fundamentally important for enhancing the carrier transport properties. Here, we investigate the frequency dispersion mechanism in a metal-oxide-semiconductor capacitor (MOSCAP) with a monolayer MoS 2 and an ultra-thin HfO 2 high- k gate dielectric. We show that the existence of sulfur vacancies at the MoS 2-HfO 2 interface is responsible for the generation of interface states with a density ( D it ) reaching ~7.03 × 10 11 cm −2 eV −1. This is evidenced by a deficit S:Mo ratio of ~1.96 using X-ray photoelectron spectroscopy (XPS) analysis, which deviates from its ideal stoichiometric value. First-principles calculations within the density-functional theory framework further confirms the presence of trap states due to sulfur deficiency, which exist within the MoS 2 bandgap. This corroborates to a voltage-dependent frequency dispersion of ~11.5% at weak accumulation which decreases monotonically to ~9.0% at strong accumulation as the Fermi level moves away from the mid-gap trap states. Further reduction in D it could be achieved by thermally diffusing S atoms to the MoS 2-HfO 2 interface to annihilate the vacancies. This work provides an insight into the interface properties for enabling the development of MoS 2 devices with carrier transport enhancement.