The effects of a series of normal alkanes (decane, dodecane, tetradecane, hexadecane, and octadecane) on the hexagonal HII structures containing dioleoylphosphatidylethanolamine (DOPE) and dioleoylphosphatidylcholine (DOPC) were studied using x-ray diffraction and osmotic stress. The alkanes affect structural dimensions and the monolayer intrinsic curvature and bending modulus. The alkane effects are chain-length dependent and are attributed to their different distribution within the HII structure. The data suggest that short-chain alkanes are more uniformly distributed within the HII hydrocarbon regions and change the curvature and bending modulus of the monolayer, whereas longer-chain alkanes appear confined more to the interstitial region and do not change the curvature and bending modulus.