This series of cyclopropenium salts was also investigated for their ability to act as synergists for the kinetic hydrate inhibitor (KHI), polyvinylcaprolactam (PVCap). In a series of high pressure rocking cell tests using a structure II-forming natural gas mixture, the hexa-butylated cyclopropenium derivative performed well, but the best synergist was the cyclopropenium derivative with a 2:1 mixture of di-iso- and di-n-pentylamino:diethylamino groups. As has been shown in previous work with the synergism of tetraalkylammonium salts with PVCap (P.C. Chua, M.A. Kelland, 2012. Energy Fuels 26, 1160), we presume that adsorption onto hydrate crystal surfaces is not the only KHI mechanism operating and that perturbing the water structure and thus the nucleation of gas hydrates with the cyclopropenium salts is also a key KHI mechanism.