A spectroscopic method, based on the interface selectivity of second-harmonic generation, is used to obtain the polarity of liquid interfaces. In this paper the second-harmonic measurement of the spectrum of the polarity indicator molecule N,N-diethyl-p-nitroaniline (DEPNA) at the air/water interface demonstrates the method. Two different approaches are used to measure the intramolecular charge transfer (CT) absorption band position of DEPNA at the air/water interface. The DEPNA CT band blue-shifts from 429 nm in bulk water (polar solvent) to 359 nm in bulk hexane (nonpolar solvent) and 329 nm in the gas phase (no solvent). At the air/water interface, the charge transfer peak band maximum occurs at 373 nm, which indicates that the polarity of the air/water interface is similar to those of the bulk solvents carbon tetrachloride and butyl ether. The DEPNA results together with the results from another solvatochromic polarity indicator molecule, ET-(30), which will be reported elsewhere, show that the polarity results of the air/water interface are general.