This paper presents a new thermodynamic calculation procedure to predict gas hydrate equilibria in the presence of sodium dodecyl sulfate (SDS) micelles. This calculation procedure employs the Poisson-Boltzmann equation to determine the electrical potential and the water chemical potential change due to the presence of micelles. This water chemical potential change is incorporated into the Zele-Lee-Holder and Lee-Holder distortion model to predict equilibrium conditions of gas hydrates in the micelle solution. The prediction results agree with experimental results within an error of 5-20%. Both results show that the equilibrium dissociation pressures of methane hydrates at SDS concentrations ranging from 250 to 10 000 ppm are not much different from those in the absence of SDS. However, the initial formation rate of methane hydrates is greatly affected by SDS. One useful finding is that there is an optimal SDS concentration for the maximum initial formation rate.