Poly(N-vinyl pyrrolidone) (PVP) containing the five-ring lactam and poly(N-vinyl caprolactam) (PVCap) containing the seven-ring lactam are well-known kinetic hydrate inhibitors (KHIs). For the first time, we have synthesized and studied the performance of poly(N-vinyl piperidone) (PVPip), containing the six-ring lactam, as a KHI. In this study, we have investigated the ability of PVPip to inhibit structure II (SII) gas hydrate formation in a high-pressure stirred autoclave. The results are compared to those of PVP and PVCap. Various polymer molecular weights have been investigated at various subcoolings and with different test methods. It was found that PVPip with a molecular weight of 4000 performed better as a KHI than PVP and worse than PVCap, both of similar molecular weights. We did not observe a statistically significant difference between the nucleation inhibition performances on SII gas hydrates with PVPip polymers with molecular weights ranging from 4000 to 85 000. However, there was evidence that the highest molecular-weight polymer gave longer delays in the growth of gas hydrate crystals before fast hydrate formation ensued.