Polymerization-induced phase separations and concurrent sol-gel transitions in the hydrolysis and polycondensation of alkylene-bridged polysilsesquioxane system resulted in the formation of co-continuous macroporous gels. Gel morphology with co-continuous macropores and micrometer-sized gel skeletons was obtained in the composition region containing much higher amount of water compared with the case with other alkyltrialkoxysilane sol-gel systems. A narrow pore size distribution and high volume fraction of macropores were obtained in the median size range from few micrometers down to 0.1μm.