This study measured the flow behaviour of whey protein isolate mixtures with cinnamic or ferulic acids. Samples were heated in a vacuum (80 °C, –0.9 atm, 280 r.p.m., 0.5 h). The flow curves of all samples showed a non- Newtonian shear thinning flow and the viscoelastic properties were typical for weak gel systems. At pH 6.0, 6.7, and 8.0, the highest shear stress values were obtained with 20, 40, and 40 mg of cinnamic acid g–1 protein, respectively. At pH 6.0, the use of ferulic acid (20 mg g–1 protein) resulted in the elevation of shear stress values, but at pH 8.0, ferulic acid caused a decrease in shear stress values in comparison to cinnamic acid. The thixotropic area (AT) was increased in mixtures containing 20–40 mg cinnamic acid g–1 protein (at pH 6.7) and 20 mg of cinnamic acid g–1 protein (at pH 6.0). Similarly, the addition of ferulic acid (40 and 20 mg g–1 protein at pH 6.7 and 6.0, respectively) caused a significant increase in AT. At pH 8.0, no significant differences in AT values were observed between samples. Such systems can be applied with reference to health promoting foods such as WPI-based desserts.