Abstract The theory of polarization spectroscopy is summarized for sequential pump and probe pulses. Polarization-labelled spectra of the 2B 2 ← 2A 1 system of NO 2 vapor have been recorded under high dispersion in the range 16700– 17000 cm −1, using a pump operated at higher frequency (ca. 18323 cm −1). These spectra exhibit signals resulting from the labelled populations in both upper and lower states of the pumped transitions. The technique is especially sensitive to transitions from the upper state label to vibrationally-excited levels of the ground state, because a population inversion then exists and the signal wave experiences gain as it propagates through the medium.