Biological milieus are highly complex and crowded environments where biomolecules are in close proximity to each other. Limited data have demonstrated that these crowded systems – largely dominated by interfacial physical chemistry – modulate the biochemical reactions of proteins which are the most abundant macromolecules inside cells. Despite evidence demonstrating that small neutral oxidants of moderate reactivity (e.g. 1O2), can diffuse freely in packed systems such as lipid bilayers and cells, slower and anomalous diffusion is expected for larger oxidants, such as peroxyl radicals (ROO•) generated during lipid or protein peroxidation. This, along with the close proximity of proteins in biological systems, may affect the propagation of oxidative damage at an inter- as well as intra-molecular level. Therefore, we hypothesized that crowding might modulate the rate and extent of protein oxidation. To test this hypothesis, we have examined model in vitro systems containing free amino acids (Trp, Tyr and Cys), and small proteins that lack, or have low numbers of these residues and measured the rate of consumption induced by ROO• in the absence and the presence of inert crowding agents. Kinetic data and mass spectrometry analyses indicate that the rate and extent of consumption of the amino acids is enhanced under macromolecular crowding conditions. Thus, for example, the rate of Trp oxidation was increased from 15.0 ± 2.1 μM/min in PBS to 30.5 ± 3.3 μM/min in the presence of dextran (60 mg/mL). These data imply an increase in the length of chain reactions, and therefore propagation of oxidative damage, from 1.9 in diluted systems to 3.8 under macromolecular crowding conditions. A better understanding of these processes may allow the development of strategies to prevent amino acid and protein oxidation in crowded environments including protein-based medicines and vaccines where concentrations of up to 100 mg protein/mL are encountered.