Quaternary ammonium ionic liquid salts (QAILs) are well known as synergists for kinetic hydrate inhibitor (KHI) polymers such as poly(N-vinyl caprolactam) (PVCap). Earlier work showed that branching of the pentyl tails of tetra(n-pentypammonium bromide to make isohexyl groups and tert-heptyl groups gave a significant improvement in the synergistic KHI performance. We have now evaluated in more detail the KHI performance of tris(tert-heptyl)-N-pentyl-1-ammonium bromide (tris(tert-heptyl)PeAB) or tris(tert-heptyl)-N-propyl-1-ammonium bromide (tris(i-heptyl)PrAB) blended with poly(N-vinyl caprolactam) (PVCap) in steel rocking cells at 130 bar. Due to tris(tert-heptyl)PeAB being sparingly soluble in water we used a water gas decane system in order to get this compound dissolved in the fluids. These QAILs were also compared to the known QAILs, tetra(n-butyl)ammonium bromide (TBAB), tetra(n-pentyl)ammonium bromide (TPAB), tetra(n-hexyl)-ammonium bromide (TnHexAB), tetra(isohexyl)ammonium bromide (TiHexAB), tetra(n-heptyl)ammonium bromide (TnHepAB), and hexa-n-butylguanidinium chloride (n-Bu6GuanCl),. Under constant cooling test conditions it was found that both tert-heptyl-based QAILs as well as TiHexAB and n-Bu(6)GuanCl gave particular good synergistic performance results. To further differentiate the ranking of these QAIL synergists, we also conducted long-term isothermal KHI performance tests on the best synergists. When PVCap was mixed with tris(tert-heptyl)PeAB it gave better performance than when mixed with TPAB, TiHexAB, or n-Bu6GuanCl in tests at two different isothermal test conditions. These results highlight the excellent synergy of tert-heptylated quaternary ammonium salts and that sparingly water-soluble but oil-soluble quaternary ammonium salts, with the correct functional groups, can still be good synergists for KHI polymers such as PVCap.