Solid acid/base properties of organosiloxane-based inorganic-organic hybrid materials are reported. The organic groups bonded to silicon in organosiloxane networks are found to synergistically affect the solid acid/base properties arising from inorganic components (see Figure). This synergistic effect may result in innovative materials with applications in fast proton conductors, selective catalysts, efficient membranes, high-sensitivity sensors, and selective absorbents.