A thermodynamic model for the calculation of hydrate formation temperatures of different hydrate formers in mixed electrolyte solutions is presented. The model uses the existing theory for the calculation of the water fugacity in single salt solutions and has been extended to mixtures of common electrolytes, i.e., NaCl, KCl and CaCl2. The present model, in contrast to previous models, needs no flash calculation procedure. The average absolute deviation of the model, using available experimental data, was about 0.4 K.