Grazing incidence X-ray diffraction (GIXD) and Brewster angle microscopy (BAM) were used to study the miscibility and phase behavior of Langmuir monolayers composed of a mixture of a saturated (stearic) and a trans-monounsaturated (elaidic) fatty acid. In contrast with suggestions from previous thermodynamic measurements, these compounds were poorly miscible in monolayers, and phase separation was always observed between domains of a liquid crystalline stearic acid-rich phase and a disordered elaidic acid-rich phase. The molecular packing density inside the ordered domains of mixed monolayers was within 2% of the density of pure stearic acid monolayers at the same surface pressure, suggesting that this phase contained at most a very small fraction of the larger elaidic acid molecules. However, the presence of this small concentration of unsaturated chains depressed the L-2 to Ov phase transition by similar to7 mN/m.