The quiescent annealing of four diffrent co-continuous polystyrene/high-density polyethylene blends of widely different viscosity ratios were examined at three different temperatures. In addition, the coarsening of a co-continuous poly(methyl-methacrylate)l high-density polyethylene blend was also studied. Since the morphology of co-continuous systems is very difficult to accurately analyze using microscopic techniques, the pore dimensions of the PS and PMMA phase are characterized, after solvent extraction, using mercury porosimetry. The volume average pore diameter is used in order to track the large pores in the system. A significant coarsening effect, as evidenced by the growth of pore size, is observed. For these uncompatibilized systems a direct relationship between pore size R and annealing time t (R similar to kt) is observed. Using a conceptual model of co-continuity, based on thin and thick rods, it is proposed that the driving force for the coarsening process is a capillary pressure effect. The differences in capillary pressure throughout the co-continuous structure result in the continuous merging of thin parts toward the thick ones. This process is confirmed through the presence of a large number of extremely thin threads in contact with very thick ones after annealing. In order to understand the factors influencing the coarsening rate we have adapted an approach used for phase separation. The thick rod is treated as a cylindrical thread which cannot breakup via a capillary instability due to the numerous branches which continuously feed it. In such a case it is proposed that the rate of growth of the distortion amplitude, dalpha/dt, taken from Tomotika's analysis for capillary instabilities, can be directly related to the coarsening rate, dR/dt. Since alpha(0)/R-0 (the ratio of the initial distortion amplitude to the initial thread radius) is found to be constant for all of the co-continuous systems studied, all of the coarsening rates for the various systems are controlled by the interfacial tension, the zero shear viscosity of the surrounding medium and Omega from Tomotika theory. An excellent correlation of this model is demonstrated for all of the systems studied. These results and the proposed mechanism also indicate that the quiescent coarsening of immiscible co-continuous blends can continue over long time periods while still maintaining co-continuity and, hence, can provide an important route toward morphology control in percolated systems. (C) 2004 American Institute of Chemical Engineers.