The thermodynamic hydrate inhibition (THI) effect induced by three different quaternary ammonium salts (QASs) namely tetraethylammonium iodide (TEAI), tetramethylammonium bromide (TMAB) and tetraethylammoniun (TEAB) on the formation of methane gas hydrates has been investigated. The THI impact is also determined in the presence of monethylene glycol (MEG) for TMAB and TEAB. The inhibition capacity is examined by finding the average suppression temperature. Experiments are performed in the pressure range of 3.45�8.3 MPa at the concentration values of 1, 5 and 10 wt. Hydrate liquid vapor equilibrium (HLVE) points are calculated by using isochoric thermodynamic cycle (T-cycle) method. COSMO-RS study is also performed to understand the THI behavior of the compounds by analyzing sigma profile. Along with it hydrate dissociation enthalpies (�Hdiss) are considered. The results revealed that inhibition effect increased with increase in concentration. TMAB showed better impact on hydrate suppression as compared to TEAB and TEAI in pure as well as in mixture with MEG. Moreover, in thermodynamic modeling section, Dickens and Quinby-Hunt model is employed to predict HLVE data. The equilibrium temperatures predicted by model are compared with the experimental results and found to be in good agreement. Hence these compounds can be employed in THI applications and flow assurance schemes. © 2019 Elsevier Ltd