Egg phosphatidylcholine small unilamellar vesicles ranging from 150 to 270 A in diameter have been studied by proton nuclear magnetic resonance (400 MHz) to investigate the relationship between phosphatidylcholine head group conformation and small changes in the vesicle radius of curvature. We find that as the vesicle size decreases, the split between the choline N-methyl resonances, corresponding to lipids residing in the outer and inner monolayer, becomes more pronounced. The increasing split is due to the dramatic upfield shift of the inner monolayer choline resonance with decreasing vesicle size. We also investigated the formation of deoxycholate- phosphatidylcholine mixed micelles by following the changes in the choline N-methyl resonances of small unilamellar vesicles with the progressive addition of deoxycholate. Our data provide additional support for the proposal by Mazer et al. [Mazer, N. A., Benedek, G. B., & Carey, M. C. (1980) Biochemistry 19, 601] of the existence of distorted bilayer structures at low deoxycholate:lipid molar ratios (<0.5 mM), which decay to mixed micelles at higher deoxycholate concentrations. Taken together, our results demonstrate that the choline N-methyl chemical shift is a sensitive indicator of head group surface area. Furthermore, we infer from our results that (1) the inner monolayer head group packing significantly influences the size limitations of a small unilamellar vesicle, (2) the inner phosphatidylcholine N-methyl chemical shift is indicative of the vesicle radius, and (3) the chemical shift of a phosphatidylcholine N-methyl residing in a planar bilayer will be very similar to that in an outer monolayer of a small unilamellar vesicle. © 1982, American Chemical Society. All rights reserved.