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We have used grazing incidence wide-angle x-ray scattering (GIWAXS) on oriented lipid multilayers to measure chain order and to examine liquid-liquid coexistence in the system DOPC/DPPC/cholesterol, a model for the outer leaflet of the cell plasma membrane. Coexistence of liquid-disordered (Ld) and liquid-ordered (Lo) domains is thought to be related to "rafts " in the cell membrane, cholesterol-rich lipid heterogeneities which provide platforms for protein sorting. Many of the methods used for measuring liquid-liquid coexistence in model membranes require a potentially perturbing probe, while x-ray scattering is probe-free. In unoriented (powder) x-ray data, scattering from the Ld and Lo phases looks very similar, whereas in GIWAXS patterns from oriented samples, these phases are easily distinguishable because of the differences in their chain orientational order. By using a simple analytical model to relate the GIWAXS data to the chain orientational distribution, we fit our data to obtain the average chain orientational order parameter, Smol. While this type of analysis has been well-used for liquid crystals, it is not commonly applied to model membrane systems. For DOPC/cholesterol and DPPC/cholesterol mixtures