A series of amine oxide and acylamide derivatives of polyethylenimine, both hyperbranched polyethylenimine (HPEI) and linear polyethylenimine (LPEI), has been synthesized. Their performance as crystal growth inhibitors of structure II tetrahydrofuran (THF) hydrate has been investigated. In general, the amine oxide derivatives gave significantly better crystal growth inhibition than the acylamide derivatives, and LPEI derivatives gave better performance than HPEI derivatives. The alkyl group that gave the best growth inhibition within the amine oxide class was n-butyl, in agreement with previous studies on amine oxides. Butylated amine oxides of LPEI were more powerful THF hydrate crystal growth inhibitors than those of HPEI at similar molecular weights. Indeed, a butylated amine oxide derivative of LPEI (molecular weight of 2500 g/mol) gave superior THF hydrate crystal growth inhibition than any previously reported chemical. The acylamide derivatives gave the best hydrate growth inhibition when the pendant groups were propyl. Larger alkyl groups gave water-insoluble polymers. In agreement with the amine oxide results, THF hydrate crystal growth inhibition is somewhat better for the LPEI acylamide derivatives. This is probably related to the geometry of the hyperbranched polymers in that some branches of any one polymer will have steric difficulty in interacting with hydrate surfaces. This steric problem does not occur in single-strand linear polymers. Acylamide derivatives with pendant isopropyl groups for a series of oligomeric ethyleneamines were also synthesized. The THF hydrate crystal growth inhibition results suggest that the shorter oligomers perform worse than equivalent polymers with molecular weights of a few thousand grams per mole. Increasing polymer molecular weights to over 10 000 g/mol lowers the crystal growth inhibition performance, suggesting that there is an optimum molecular weight for this class of polymer.