A series of alternating peptides with glycine-N-substituted valine dipeptide repeating units were synthesized. The N-substitution included methyl, ethyl, n-propyl, and hydroxyl groups. The performance of the polypeptides as kinetic hydrate inhibitors (KHIs) was evaluated in high pressure rocking cells with a synthetic natural gas mixture using the slow constant cooling method. All the polypeptides gave better KHI effect than a low molecular weight poly(N-vinylpyrrolidone). The polypeptide with N-substituted n-propyl groups, PA-Pep, performed the best, the same as a commercial poly(N-vinyl caprolactam) (PVCap) at the same concentration. The KHI performance of PA-Pep improved as the concentration of the solution was increased from 1000 to 5000 ppm. At 5000 ppm the average (10 experiments) onset temperature for hydrate formation was 9.8 degrees C (ca. Delta T = 10.4 degrees C).