The relative phase and amplitude ratio between the ssp and ppp polarization combinations of the vibrational sum-frequency generation (SFG) response can be uniquely and accurately determined by the polarization null angle (PNA) method for orientational analysis. With PNA measurement we show that the -CN group in the 4-n-pentyl-4'-cyanoterphenyl (5CT) Langmuir monolayer is tilted around 25 degrees +/- 2 degrees from the interface normal, while that in the 4-n-octyl-4'-cyanobiphenyl (8CB) is tilted around 57 degrees +/- 2 degrees, consistent with the significant differences in the phase diagrams and hydrogen bonding SFG spectra of the two Langmuir monolayers reported in the literature. (C) 2013 Elsevier B.V. All rights reserved.