The structure and composition of the mixed monolayer of hexadecyltrimethylammonium bromide and benzyl alcohol adsorbed at the air-water interface has been measured by using specular neutron reflection. The alkyl chain conformation of the hexadecyltrimethylammonium bromide at a surfactant concentration of 2 x 10(-3)M is not significantly altered by the addition of benzyl alcohol. However, the addition of benzyl alcohol draws the hexadecyltrimethylammonium bromide alkyl chain closer to the aqueous subphase : a similar change is observed in the mixed monolayers of hexadecyltrimethylammonium bromide-hexaethylene glycol monododecyl ether and of sodium dodecyl sulfate-dodecanol, The distribution of the benzyl alcohol molecules in the interface is found to be centered on the third methylene group from the headgroup of the alkyl chain of the hexadecyltrimethylammonium bromide cationic surfactant.