Mixed monolayers of CTAB (hexadecyltrimethylammonium bromide) and tetradecane were formed at the air-water interface by addition of small amounts of tetradecane to the surface of a CTAB solution below the critical micelle concentration. Ellipsometry and sum-frequency spectroscopy were used to demonstrate that these mixed monolayers exhibit a first-order phase transition from a conformationally disordered to a conformationally ordered state as the temperature is lowered. The phase transition occurs ca. 11 degrees C above the bulk melting point of tetradecane.