Methods of microscopic observation and macroscopic characterization have been developed for determining the co-phase continuity in immiscible binary blends. After selective dissolution of the component polymers, the morphologies of microscopic observation are consistent with the results of macroscopic observation and weight percentage determination. By using these methods, the relationship between co-phase continuity, composition and blending time has been explored for two immiscible binary polyblends with different viscosity ratios (lambda), polyamide 6/polyethersulfone (PA/PES, lambda = 0.03) and poly(butylene terephthalate)/polystyrene (PBT/PS, lambda = 1). Both blend systems show a similar dependence of co-phase continuity on the composition and mixing time. That is at short mixing time (for example, 2 minutes), the co-phase continuity takes place in a wide composition range. With increasing blending time, the composition range of co-phase continuity becomes narrow, and finally shrinks to one point. After a long enough mixing time the co-phase continuity region will occur only at a volume fraction of 50/50, no matter what the viscosity ratio of the blend is.