Various mixtures of two chemicals have been tested for their ability to prevent tetrahydrofuran (THF) hydrate crystal growth and compared to their ability as gas hydrate kinetic hydrate inhibitors (KHIs) using a structure-II-forming natural gas mixture, the same structure Obtained with THF hydrates. The THF hydrate results for mixtures of poly(N-vinylcaprolactam) (PVCap) with tetra-n-butylammonium bromide (TBAB), tetra-n-butylphosphonium bromide (TBPB), or hexa-n-butylguanidinium chloride (Bu(6)GuanCl) showed clear synergy effects in two different types of tests: an isothermal test at -0.5 degrees C and a variable temperature test. In these tests, the key parameters are the lowest concentration of a mixture and the highest subcooling for which complete THF hydrate inhibition is observed, respectively. The synergistic similarities between the THF and previously obtained structure-II-forming gas hydrate results suggest that the dominant inhibition mechanism operating in these mixtures in the gas hydrate system is crystal growth inhibition. Poor THF hydrate crystal growth inhibitors, such as tetra(n-hexylanimonium bromide) (THAB), showed poor synergy with PVCap in THF hydrate tests but do show synergy with the gas hydrate system. These results indicate that another mechanism besides crystal growth inhibition is operating, in the gas hydrate system. We suggest that this other mechanism, is some form of nucleation inhibition or particle destabilization mechanism. This is discussed in light of other evidence from laboratory experiments and computer modeling. Nucleation inhibition in gas hydrate systems may be occurring not just for THAB but as a secondary mechanism for the other compounds tested in this study.