A method for inhibiting the formation of clathrate hydrates in a fluid having hydrate-forming constituents involves treating the fluid with an inhibitor comprising a substantially water soluble polymer having a polymer backbone with a pendant C1-C3 alkyl group; the polymer having an average molecular weight between about 1,000 and about 6,000,000. Preferably, the pendant alkyl group is a methyl group. The rate of nucleation, growth, and/or agglomeration of gas hydrate crystals in a petroleum fluid stream is reduced using the alkylated polymer backbone inhibitor, thereby inhibiting the formation of a hydrate blockage in the pipe conveying the petroleum fluid stream. Test results are disclosed which unexpectedly show that alkylating the polymer backbone with a methyl group will produce a subcooling for the alkylated polymer that is at least about 2 °F (about 1.1 °C) greater than its nonalkylated counterpart. The inhibitor can be used conveniently at low concentrations, for example in the range of from about 0.01 % to about 5 % by weight of the water present in the oil or gas stream, to effectively treat a petroleum fluid having a water phase.