Langmuir monolayers of straight-chain and branched hexadecanol and eicosanol mixtures were previously studied using surface pressure- area isotherms, Brewster angle microscopy, and interfacial rheology. In this paper, we investigate the structure of these fatty alcohol mixtures using these previous results together with X-ray diffraction and reflectivity measurements, which provide a better understanding of the structure of the monolayer in terms of the phase segregation and location of branched chains. For eicosanol below 25 mN/m, the branched chains are incorporated into the monolayer, yet they are phase-separated from the straight chains. At higher surface pressures, the branched chains are expelled from the monolayer and presumably form micelles or some other aggregate in the subphase. In contrast, the hexadecanol branched chains are not present in the monolayer at any surface pressure. These behaviors are interpreted with the help of the X-ray measurements and density profiles, and are explained in terms of straight-chain flexibility. We will discuss the effect of the monolayer structure on the surface shear viscosity. These studies provide a deeper understanding of the structure and behavior of amphiphilic mixtures, and will ultimately aid in developing models for lipids, micelle formation, and other important biological functions.