The gastric tract may be the first site where food is exposed to postprandial oxidative stress and antioxidant activity by plant micronutrients. After food intake, dietary iron, dioxygen, and emulsified lipids come into close contact and lipid oxidation may take place. This study investigated lipid oxidation and its inhibition by dietary polyphenols in gastric-like conditions. Lipid oxidation induced by heme and nonheme iron was studied in acidic sunflower oil-in-water emulsions. The emulsifier type (bovine serum albumin, phospholipids), pH, and iron form were found to be factors governing the oxidation rates. Quercetin, rutin, and chlorogenic acid highly inhibited the metmyoglobin-initiated lipid oxidation in both emulsified systems at pH 5.8. Additionally, quercetin inhibited nonheme iron-initiated processes, while it was inefficient with hematin as an initiator. The presence of human gastric juice did not influence lipid oxidation, although it diminished the antioxidant activity of phenolics. Model emulsions may thus be valuable tools to study the gastric stability of polyunsaturated lipids.