The mechanisms of phase separation during membrane preparation were investigated by light scattering during immersion of cellulose acetate casting solutions in water/acetone coagulation baths. Data obtained for casting solutions with a concentration range (11-17 wt% cellulose acetate in acetone) normally chosen for membrane preparation, coagulated in baths containing up to 65 wt% acetone, could be well fitted with the linear Cahn theory for spinodal decomposition at least in the early stages of phase separation. Nucleation and growth was observed for casting solutions with polymer concentration lower than 6 or higher than 25 wt%. The morphology of membranes obtained by different mechanisms was compared.