The physical properties of semi-clathrate hydrates formed with tetra-n-butylammonium fluoride (TBAF) and tetra-n-butylammonium bromide (TBAB) in the temperature range of 83-263 K were studied using terahertz time-domain spectroscopy (THz-TDS). From our measurements, the semi-clathrate hydrate of TBAF shows the general trend of slightly higher refractive indices than the TBAB semi-clathrate hydrate. Furthermore, significant differences in the absorption coefficients and dielectric constants between the TBAF and TBAB semi-clathrate hydrates are discovered. These originate from fundamental structural/compositional differences between the hydrates which are related to their gas storage capacities. (C) 2013 Elsevier B. V. All rights reserved.