Borate–crosslinked galactomannan gels are commonly used as fracturing fluids. These fluids are reversible gels having crosslink junctions that easily dissociate causing the gels to be both thermal and shear thinning. Rheological measurements and 11B NMR analysis were used to examine the nature of these crosslink junctions. The NMR data suggests that a 1:1 borate polymer complex forms in abundant quantities. The primary structure of the crosslink junction may be an ionic association between the complex and adsorbed cations on the second polymer chain. Steady and oscillatory shear measurements were also made to examine the properties of the crosslink junction.