Fairly uniform poly(styrene)-poly(methyl methacrylate) (PST-PMMA) composite microspheres were prepared by employing an SPG (Shirasu Porous Glass) membrane emulsification technique. A mixture of PST, PMMA and cosurfactant (lauryl alcohol, LOH) dissolved in dichloromethane (DCM) was used as a dispersed phase, and an aqueous phase containing poly(vinyl alcohol) and sodium lauryl sulfate as a continuous phase. It was found that LOH volume affected significantly the morphologies of the composite particles apparently. The effects of LOH volume, PMMA/PST ratio, and polymer concentration on morphologies of the composite particles were investigated. When polymer concentration in the dispersed phase was lower, thermodynamically equilibrium morphology was obtained in most cases. Morphology changed to inverted core-shell from core-shell or hemisphere as PMMA/PST ratio decreased. When polymer concentration was high, various interesting non-equilibrium morphologies, such as microdomain, multiplet, inverted core-shell-shell particles, were observed. Two- and three-component models, which were developed based on Sundberg`s theory, were used to calculate the equilibrium morphologies for the cases with and without adding LOH, respectively. An agreement was obtained between experimental and calculated results.