We studied how protein fibrils (1 ¿m in length, prepared by heating at pH 2) can be used to modify structural properties of concentrated whey protein isolate (WPI) solutions. Using transmission electron microscopy, flow-induced birefringence and Thioflavin T fluorescence, we observed that the fibrils become shorter upon increasing the pH, and clusters were present at pH 5¿7. For pH 3.5 and 7, the effect of adding fibrils to WPI solutions was studied. Different behaviour was observed, which makes this system rather complex. Rheological measurements showed that the presence of fibrils induced shear thickening (only at pH 3.5) and shear thinning behaviour, and resulted in an increase in viscosity and gel strength. Unstable flow regimes were observed for both pH values, which hint in the direction of phase separation. For pH 7, a phase separated dispersion was observed in the macrostructure after gelation using scanning electron microscopy.