Despite the evidence supporting a relationship between phenolipid's alkyl chain length and their interaction/affinity with membranes, such a connection has not been directly evaluated. In this study, the membrane interactions of rosmarinic acid, and its corresponding alkyl esters having 4 carbon (RC4), 8 carbon (RC8), 10 carbon (RC10), and 16 carbon (RC16) have been investigated via small angle X-ray scattering (SAXS) approaches in model membrane liposomes. The results of this work show significant differences between the tested phenolipids with respect to their interaction and location within the lipid bilayer. Both membrane affinity and penetration depth follow a nonlinear behavior. They have been shown to increase as a function of increasing alkyl chain length until a threshold is reached for medium alkyl chain, beyond which any lengthening of the chain results in a collapse. RC8 show the highest affinity for the membrane, whereas RC10 constitute a critical chain length from which the penetration depth decreases. The X-ray scattering analyses seem to perfectly correlate with the phenolipids' activities (antioxidant and membranes crossing) previously observed both in biological system and in formulated lipid dispersions. Practical applications: Phenolic compounds are potent antioxidants that may play an important role as well in biological system as in formulated lipid dispersions. However, their activity is often limited by low solubility or bioavailability. Lipophilization of phenolic compounds may increase their solubility in lipophilic matrices, conferring better antioxidant protection for food or nonfood application. In biological system, lipophilization may contribute to easier penetration of antioxidants through lipid bilayer of cell membranes, which can significantly increase their bioavailability. Despite the evidence supporting a relationship between phenolipid's alkyl chain length and their interaction/affinity with membranes, such a relationship has no