Fluids pumped in hydraulic fracturing operations constitute highly viscous gels achieved via cross-linking of the polysaccharide derivative hydroxypropyl guar (HPG), e.g., with titanium or zirconium complexes. In this study, the mechanism underlying the cross-linking effect was clarified experimentally for the titanium system. It was found that the cross,linking effect is not based on a ligand exchange reaction with the cis-hydroxy functionalities present in HPG but instead relies on TiO2 nanoparticles resulting from hydrolysis of the Ti complexes. For example, 6 nm titania nanospheres (anatase polymorph) synthesized via acid hydrolysis of tetraisopropyl orthotitanate can increase the viscosity of a HPG solution by a factor of 25. However, this effect was observed only at pH = 2-4 where the nanoparticles are stable. At higher pH values, electrostatic repulsion between the nanoparticles decreases, resulting in agglomeration as was observed via zeta potential and pH-dependent particle size measurement. Such agglomerates exhibit a lower surface area and thus produce a weaker or no cross-linking at all. Similar effects were obtained from TiO2 particles when their diameter was increased stepwise from 6 to 14 nm. Minor additions of citric acid can stabilize the nanoparticles even at pH 5-11, thus allowing efficient cross-linking of HPG also in an alkaline environment while excessive amounts of citric acid (>5 mmol/g TiO2) retard the cross-linking process. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy revealed that the cross-linking mechanism is based on the formation of hydrogen bonds between OH groups present on the surface of titania and along the galactomannan chain of HPG. The study confirms that TiO2 nanoparticles represent the active species responsible for the cross-linking of HPG in fracturing fluids when titanium complexes are used.