A series of water-soluble polymers has been investigated for comparison of their potential as kinetic hydrate inhibitors (KHIs) for both structure I (SI)- and structure II (SII)-forming gas mixtures. A slow constant cooling test method and steel rocking cells have been used for these experiments. The average hydrate onset temperatures (T-o) have been used to rank the polymers according to their KHI ability, with the lower T-o values giving a higher rank. The object of the study was a comparison of the KHI performance for SI versus RI inhibition for a range of polymers and not a study to find the optimal polymer for each hydrate structure. The most interesting comparative results were the large deviations of the ethyl derivative of poly(N-alkylglycine) for the two hydrate systems, where this polymer performed very well with the SI-forming gas but had significantly less effect on the SII-forming gas. Also, poly(2-ethyl-2-oxazoline) (PEtOx) and poly(isopropenyloxazoline)-01 (PiPOx-01) ranked better with SI hydrates in comparison to the SII hydrate system. In general, the polymers that work well on the SI-forming gas work well on the SII-forming gas as well. KHI mechanistic insights from this study were discussed. Although other KHIs outside this study may be available with better performance, we found that the best KHIs for RI hydrates were two N-vinylcaprolactam polymers in the synergistic solvent 2-butoxyethanol, and the best KHIs for SI hydrates were a N-isopropylacrylamide homopolymer and the same two N-vinylcaprolactam polymers.