The effect of membrane thickness on surface morphology has been studied for the case of polyphenylene oxide (PPO) membranes cast from solutions of PPO in trichloroethylene (TCE). Both roughness and nodule size decrease with increasing membrane thickness up to a certain point after which they begin to increase. Minima in roughness and nodule size are observed for 9-11 mu m thick films. These minima depend on initial polymer concentration in the casting solution. At a membrane thickness greater than 11 mu m, super nodular aggregates are formed. A mechanism based on Marangoni and Rayleigh number is presented.