The high resistance to water evaporation exhibited by monolayers of octadecanol is markedly reduced if small quantities of cholesterol are present, an effect that mirrors the behavior of contaminants in octadecanol monolayers. Surface pressure-area isotherms show that octadecanol and cholesterol are almost (but not entirely) immiscible, Grazing-incidence synchrotron X-ray diffraction (GIXD) of the floating monolayers shows that cholesterol produces no diffraction pattern and thus is amorphous, octadecanol gives a clear diffraction peak which can be deconvoluted into a major and a minor peak corresponding to a slightly distorted hexagonal packing of the alkyl chains, and the alcohol-rich mixtures give diffraction patterns like that of pure octadecanol. The latter result shows that cholesterol does not enter the octadecanol domains in the mixed monolayers. The evaporation data are explained by permeation through the irregularly packed domain boundary regions in a pure octadecanol monolayer and by the rapid increase in the size of these regions when cholesterol is added.