Quaternary ammonium salts (QASs) such as tetrabutyl ammonium bromide (TBAB) and fluoride (TBAF) can form semiclathrate hydrates with water molecules at atmospheric pressure. Recently, the semiclathrate hydrates have attracted more and more interests for their wide applications, such as the storage and transportation of natural gas or coal bed methane, separation of low-boiling gases and carbon dioxide sequestration at mild conditions. In this work, the modeling investigations were conducted for the TBAB and TBAF semiclathrate hydrates with different gases such as CH4, CO2, N-2, and their mixtures. The Chen-Guo hydrate model was extended to the semiclathrate hydrate systems. The thermodynamic properties of the gas phase were described with the Patel-Teja equation of state (PT EOS). The PT EOS combined with a Debye-Huckel electrostatic contribution term was chosen to describe the thermodynamic properties of the electrolyte solution. The parameters in the EOS for ions were determined by fitting the thermodynamic properties of the electrolyte solution. The parameters of TBAB and TBAF in the hydrate model were obtained from correlating the equilibrium data of semiclathrate hydrates. The extensive tests show that the proposed model results are in acceptable agreement with the experimental data on phase equilibria of semiclathrate hydrate systems. (C) 2016 Elsevier B.V. All rights reserved.