The synergistic effect,of hexa-n-butylguanidinium chloride (n-Bu(6)GuanCl) and related compounds in different ratio Mixtures with a series of three commercially available N-yinylcaprolactam-based polymer kinetic hydrate inhibitors (KHIs) against structure-II-forming synthetic natural gas hydrates has been evaluated by carrying out constant cooling and isothermal KHI screening experiments in high-pressure rocking cells. Formulations containing n-Bu(6)GuanCl/poly(N-vinylcaprolactam) with, weight ratios of 4:1, 9:1, and 14:1 at 3000 ppm total concentrations were found-to give the best gas hydrate inhibitory performances, with average onset temperatures (T-o) of 3.4, 3.1, and 3.3 degrees C, respectively. In comparison, 3000 ppm of poly(N-vinylcaprolactam) alone gave an average T-o of 7.7 degrees C. Using 9% white spirit lowers the T-o values of the best synergistic mixtures by 2.1 degrees C. However) this effect is at least partly duo to shifting of the hydrate phase equilibria to a lower equilibrium temperature and higher equilibrium pressure. n-Bu6GuanCl Was found to be a better synergist than n-Bu(6)GuanCH(3)COO, n-Bu(6)GuanHCOO, and diethyl-di-n-peritylguanidinium chloride [Et-2(n-Pe)(4)GuanCl] when used with poly(N-vinylcaprolactam) for gas hydrate kinetic inhibition. The synergistic-capacity of n-Bu(6)GuanCl is dependent upon the chemical structure of the N-vinylcaprolactam-based polymer, although the molecular weight differences may also play a part. A mixture of 300 ppm of high-cloud-point Nvinylcaprolactam-based terpolymer with 2700 ppm of n-Bu(6)GrianCl lowered the average To by 8.7 degrees C compared to using 3000 ppm of this terpolymer alone. In long-term isothermal experiments at 8 degrees C subcooling and 30 bar, 5000 ppm of the synergistic mixture Bu(6)GuanCl/poly(N-Vinylcaprolactam) in a 9:1 weight ratio showed a hold time of over 13.6 days. Long hold times were also achieved with various ratios of n-Bu(6)GuanCl/N-vinylcaprolactam/N-vinylpyrrolidone copolymer or n-Bu(6)GuanCl/N-vinylcaprolactam terpolymer mixtures.