A series of hexaalkylguanidinium salts have been synthesized, either from urea using the well-known Vilsmeier route or in a one-pot alkylation reaction using guanidinium chloride. The hexaalkylguanidinium salts are water-soluble up to the hexa-n-butyl derivative but sparingly soluble for the hex-n-pentyl or hexa-isopentyl derivatives. The ability of hexaalkylguanidinium salts to inhibit the growth of a tetrahydrofuran (THF) hydrate crystal has been investigated. The hexabutylated derivative gave excellent crystal growth inhibition, superior to the performance of the quaternary ammonium salt tetra(n-butyl)ammonium bromide (TBAB) and close to that of tetra(n-pentyl)ammonium bromide (TPAB). As the alkyl group is reduced in length to propyl and ethyl, the inhibition performance drops off radically. The superior inhibition performance of hexabutylguanidinium bromide was illustrated by testing its ability as a synergist for the well-known kinetic hydrate inhibitor poly(N-vinylcaprolactam) (PVCap). In high-pressure steel rocking cell and steel autoclave experiments using a natural gas mixture giving preferentially structure II gas hydrate, hexabutylguanidinium bromide dearly outperformed both TBAB and TPAB as synergists. Guanidinium chloride and hexabutylguanidinium bromide were shown to be poor antinucleator KHIs when used alone.