The hydrate inhibition effect of three kinetic inhibitors (inhibex 301, 501, and 713) was assessed from (CH4 + C-2 H-6 + C-3 H-8) gas mixture + brine systems using a high pressure sapphire cell. The onset time of hydrate formation was determined by visual observation method and pressure drop profile method, respectively. The experimental results demonstrated that the onset time was able to be determined by the visual observation method all the time while the pressure drop profile method failed to detect the onset time clearly and correctly at lower temperatures. In some cases, the initial appearance of hydrate crystals cannot induce a clear break in the pressure-lime relationship curve. The onset time measured by the visual observation method is usually shorter than or at least the same as that determined by the pressure drop profile method. The inhibiting effect on the growth of hydrate crystals can be shown by the difference of the onset time obtained by the two methods. The maximum tolerated subcooling of each inhibitor was also investigated based on the onset time. It was found that inhibex 301 behaves as the best inhibitor that can tolerate the maximum subcooling of 8.3 K at 0.5 wt% and 10.6 K at 1.0 wt%, respectively. The maximum subcooling for inhibex 501 is 6.8 K at 0.5 wt% and 6.6 K at 1.0 wt%, respectively. Inhibex 713 has relatively poor inhibiting effect among the three inhibitors with the maximum subcooling of less than 3.5 K at 0.5 wt% and 5.1 K at 1.0 wt%, respectively. (C) 2010 Elsevier B.V. All rights reserved.