Rotational vibrational. ne structure and transition dipole moment of NO2 is measured using Doppler free saturation spectroscopy with an external grating cavity quantum cascade laser (QCL). The QCL wavelength is calibrated using a 310 cm long internally coupled Fabry-Perot interferometer. We obtain a frequency splitting of 139.68 +/- 0.06 MHz (0.0047 cm(-1)) between the spin doublets P-4(+/-) (17) of 000 ? 001 transition of NO2. The resolution of the QCL based saturation spectrometer is limited by the QCL line-width of similar to 3.99 MHz (similar to 0.00013 cm(-1)) deduced from the half width of the Lamb dips. The Lamb dip spectroscopy is utilized to obtain a vibrational dipole moment of similar to 0.37 Debye for the P-4(+/-) (17) transitions. (C) 2008 Elsevier B. V. All rights reserved.