Using steady-shear rheometry in combination with high-pressure B-11 nuclear magnetic resonance spectroscopy (B-11 NMR), we have found that gels formed from water-soluble polymers containing vicinal hydroxyl groups cross-linked with various boron-containing compounds undergo significant structural changes that result in a pronounced loss of viscosity when placed under pressure. Importantly, gels from other cross-linking agents tested, including Ti(IV) and Zr(IV), did not show this loss in viscosity. The experimental study probed pressure-induced changes to both galactomannan and polyvinyl alcohol (PVA) gets cross-linked with either aryl boronic acids or alkali metal boron-containing salts using pressure conditions that ranged from atmospheric to 680 bar and temperatures that ranged from 20 to 65 degrees C. Significantly, the pressure-induced losses in viscosity and, to a somewhat lesser extent, the concomitant pressure-induced B-11 NMR spectral changes were found to be reversed upon lowering the pressure.