Aqueous dispersions of lightly sulfonated polystyrene (SPS) that was neutralized by ammonia liquor were prepared. The variations of the conductivity and viscosity of SPS ionomer solution during the emulsification process and the effect of ionic contents on the phase inversion in the emulsification process were studied. The turning points in conductivity curve and the maximum viscosity were considered as the characters of phase inversion in the process of emulsification. The mechanism of phase inversion was inferred.