The phase behavior of lipid bilayered micelles ('bicelles') (dimyristoyl-phosphatidylcholine, DMPC/dihexanoylphosphatidyl-choline, DHPC 2.6/1) has been studied by small angle X-ray scattering and P-31 NMR. Below 3% w/v the bilayers are arranged in tightly packed stacks. At intermediate concentrations single units are observed, whereas at 24% w/v and higher, weak stacking occurs again. The DMPC/DHPC ratio in the bicelles strongly increases at low concentration, which is correlated with an increase in the bicelle size and stacking. The increase of the order parameter in a magnetic field is related to the stack formation. Below 297 K there is no stacking at any concentration and no magnetic alignment.