Abstract Measurements of pressure-broadening and cross-relaxation rates of rotation–inversion transitions in the ν 4 band (1470–1600 cm −1 ) of NH 3 perturbed by CO 2 are reported. Pressure-broadening parameters were measured at a low pressure of CO 2 (0–180 Torr). At high pressure, the two lines of the doublet broaden as the pressure is increased, but with a markedly lower rate than at low pressure, until they overlap. Such an effect is due to collision cross-section relaxation between the two lines of the inversion components of the doublet. Detailed modeling of the spectral lines permits us to determine pressure-broadening coefficients for 41 transitions, and cross-relaxation parameters for 10 doublets. The results show a decrease of the pressure-broadening parameters with the J quantum number. We compared our results with other previous measurements obtained in the far-infrared and ν 2 regions, and for several foreign gases such as He, H 2 , and N 2 .