Comparison of small-angle X-ray diffraction patterns of Langmuir-Blodgett layers of 1,2-dipalmitoyl-sn-glycero-3-phosphatidic acid on different substrates reveals a small range of values for the monolayer thickness. These are close to the length of the lipid molecule. Polarized attenuated-total-internal-reflection Fourier transform IR spectroscopy shows a high degree of order in the multilayer film, with the alkyl chains approximately parallel to the substrate normal. Together, these results suggest either a very small molecular tilt angle or slight interdigitation of the layers.