Rosemary (Rosmarinus officinalis) extracts are widely used in the food, nutraceutical and cosmetic areas. Their major bioactive components have shown antioxidant, antimicrobial, anti-inflammatory, antitumorigenic and chemopreventive activities. In this work, the bioactive compounds deriving from rosemary leaves (carnosol, CAR; carnosic acid, CA; rosmadial, RAL; genkwanin, GW; rosmarinic acid, RA) were isolated and their effects on the phase behaviour of model membranes were studied by several complementary biophysical techniques. All diterpenes studied, and specifically CAR, decreased the hydrophobic interactions between acyl chains, as well as broadened and shifted the phospholipid transition to lower temperatures into dimyristoylphosphatidylcholine (DMPC) membranes. In addition, all diterpenes and genkwanin increased the lipid order of fluid DMPC membranes, exhibiting CAR and RAL the strongest membrane-rigidifying effect. The diterpenoids, especially CA and RAL, promoted the formation of hexagonal-H(II) phases at low temperatures in dielaidoylphosphatidylethanolamine (DEPE) membranes which exhibited a smaller tube-to-tube distance compared to pure phospholipid. These diterpenes were also able of promoting isotropic structures in DEPE membranes which consisted of non-periodically ordered lipid structures as demonstrated by X-ray diffraction. In contrast, minor effects were observed by rosmarinic acid. In conclusion, diterpenes and genkwanin from rosemary show membrane-rigidifying effects which may contribute to their antioxidant capacity through hindering diffusion of free radicals.