Lipid oxidation is a major cause for the degradation of the sensory and nutritional quality of many food products. In such multicomponent systems, lipid oxidation often occurs simultaneously with co-oxidation phenomena, which affect other molecules such as proteins. Lipid and protein oxidation both involve the formation of free radicals and carbonyl reactive species. Both phenomena are closely interrelated, notably because of the presence of interfaces and colloidal structures where proteins and lipids get into contact. However, the sequence of the involved reactions still remains unclear. We describe a coupled experimental approach to concomitantly assess lipid oxidation and protein modifications in protein-stabilized oil-in-water emulsions. Although both reactions were timely related, early protein modifications, monitored through measurements of tryptophan fluorescence, were detected prior to the development of the lipid oxidation markers. The nature of the involved protein modifications was then further investigated, for both interfacial and non-adsorbed proteins. Proteins located at the oil-water interface underwent extensive modifications, such as aggregation, carbonylation and loss in solubility. Conversely, non-adsorbed proteins in the aqueous phase of emulsions were hardly modified, and had a protective effect against lipid oxidation.