We have evaluated the synergism of five tetraalkyl phosphonium bromide salts with N-vinylcaprolactam based polymers by carrying out high pressure rocking cell experiments with a structure II-forming natural gas mixture. (n-Pe)(4)PBr was shown to be a superior synergist than (n-Bu)(4)PBr when used in combination with three different N-vinylcaprolactam based commercial polymers used in kinetic hydrate inhibitor formulations. These are poly(N-vinylcaprolactam) (PVCap), a N-vinylcaprolactam:N-vinylpyrrolidone 1:1 copolymer, and a N-vinylcaprolactam terpolymer. (n-Pe)(4)PBr and (n-Pe)(4)NBr were found to have very similar synergistic capacities and to perform better (n-Bu)(4)NBr when used with PVCap. (n-Bu)(4)PBr and (n-Bu)3(iso-Hex)PBr were better synergists than (n-Bu)(4)NBr with PVCap. They exhibited the same synergistic performances with PVCap even though (n-Bu)(3)(iso-Hex)PBr had been shown to be a better tetrahydrofuran (THF) hydrate crystal growth inhibitor. (n-Bu)(3)hexadecylPBr showed a synergistic performance similar to that of (n-Bu)(4)NBr with PVCap. Phe(4)PBr showed a poor performance as synergist with PVCap. This was in line with a poor THF hydrate crystal growth performance previously reported. By changing the weight ratios of (n-Pe)(4)PBr and PVCap at the same total weight concentration of 3000 ppm, it was found that the onset temperatures (T-o) decreased with increasing amount of (n-Pe)(4)PBr. The lowest To was reached for the ratio 29:1 ((n-Pe)(4)PBr:PVCap). Thus, excellent KHI performance can be obtained with as little as 100 ppm of KHI polymer as long as there is sufficient amount of a good synergist.