Abstract <p>Activation of the process of phospholipid peroxidation in the presence of divalent iron ions can cause cell death. It is generally accepted that the addition of different types of antioxidants should lead to the inhibition of this process. The aim of this paper is to study the effect of the carotenoid astaxanthin as an antioxidant on phospholipid peroxidation induced by iron ions in the composition of nano- and microemulsions in which the particle sizes are about 700–900 and 110–130 nm, respectively. It is shown that astaxanthin in the absence of iron ions reduced the malondialdehyde content in micro- and nanoemulsions. However, in the presence of iron ions, in the case of microemulsions, astaxanthin at a concentration of 30 μM reduces the content of malondialdehyde, and at a concentration of 5 μM, it has a prooxidant effect. Astaxanthin has no effect in the composition of the nanoemulsions. It is found that in the presence of iron ions, astaxanthin is bleached, undergoing degradation and oxidation. In the case of nanoemulsions, the bleaching rate constant is significantly higher than in microemulsions: 18.5 × 10³ M^–1 min^–1 and 2.1 × 10³ M^–1 min^–1, respectively. Thus, in the composition of nanoemulsions, astaxanthin quickly degrades and oxidizes (within 30 min), and in the composition of microemulsions, this process is slower. Possible mechanisms of interaction of astaxanthin with iron ions are discussed, which include both the formation of astaxanthin radicals and a nonradical complex with iron ions.</p>