The efflux transporter P-glycoprotein (P-gp) significantly modulates drug transport across the intestinal mucosa, strongly reducing the systemic absorption of various active pharmaceutical ingredients. P-gp inhibitors could serve as helpful tools to enhance the oral bioavailability of those substances. As a membrane-associated protein P-gp is surrounded and influenced by phospholipids. Some synthetic phospholipids have been found to strongly reduce P-gp's activity. In this study two representative phospholipids, 1,2-dioctanoyl-sn-glycero-3-phosphocholine (8:0 PC) and 1,2-didecanoyl-sn-glycero-3-phosphocholine (10:0 PC), were compared with Tween® 80 and Cremophor® EL, both commonly used surfactants with P-gp inhibitory properties. Their influence on the cellular transport of the P-gp substrate rhodamine 123 (RH123) was examined using Caco-2 cell layers. In addition, fluorescence anisotropy measurements were performed in order to investigate their effect on membrane fluidity. Finally, we compared the phospholipids with Tween® 80 and the competitive P-gp inhibitor verapamil in an in vivo study, testing their effects on the oral bioavailability of the P-gp substrate drug ritonavir. Both phospholipids not only led to the strongest absorption of RH123, but a permeability enhancing effect was detected in addition to the P-gp inhibition. Their effects on membrane fluidity were not consistent with their P-gp inhibiting effects, and therefore suggested a more complex mode of action. Both phospholipids significantly increased the area under the ritonavir plasma level curve (AUC) within 150 min by more than tenfold, but were inferior to Tween® 80, which showed superior solubilizing effects. Finally, these phospholipids represent a novel substance class showing a high permeabilization potential for P-gp substrates. Because of their physiological structure and intestinal degradability, good tolerability without systemic absorption is expected. Formulating P-gp substrates with an originally low oral bioavailability is a difficult task, requiring concerted interplay of all excipients. P-gp inhibiting phospholipids offer a new tool to help cope with these challenges.