Phenols are widely present in plants and often are co-extracted in plant protein extracts, while such components could influence the protein's interface and foam stabilising properties. In this study, the influence of rapeseed phenol sinapic acid (SA) on the interfacial and foaming properties of a well characterised model protein, whey protein isolate (WPI), was investigated. WPI formed strong viscoelastic interfacial layers, and addition of SA reduced the surface dilatational modulus by 25%. Turning SA into its active oxidised form in the WPI-SA mixtures led to protein aggregation, resulting in a further decrease of the modulus by 40%. Removal of unbound phenols induced a slight increase of the dilatational modulus, but the interfacial layer strength did not fully recover to that made with pure WPI, suggesting binding of phenols to proteins, and thus influencing the protein interface stabilising properties. Foams stabilised by WPI-SA mixtures had a shorter foam half-life time (130-180 min) than foams stabilised by pure WPI (260 min). Our data are thus in line with the observation that a lower surface dilatational modulus leads to a lower foam stability. Yet, the foam stability did not recover to the original values of pure WPI-stabilised foams after removal of unbound phenols. In conclusion, the presence of SA resulted in a decrease in interfacial layer strength and foam stability. We conclude that in producing protein extracts from rapeseed or other phenol-rich plant sources, the presence and oxidation of phenols should thus be considered.