Abstract The lineshapes of the ssR (4, 2), ssQ (6, 4), and aaP (2, 1) transitions in the ν 2 band of 15 NH 3 were recorded at several pressures by means of a computer-coupled infrared laser microwave two-photon spectrometer. The lowest pressure lineshapes were Gaussian to high precision with Doppler widths within 0.1% of the calculated values. Plots of Lorentz widths vs. pressure obtained by assuming the lineshapes to be Voigt profiles gave negative intercepts. The data were reanalyzed under the assumptions of the soft-collision and hard-collision models of collisional narrowing. The resulting pressure-broadening parameters are 16.6, 18.1, and 16.5 MHz/Torr for the ssR (4, 2), ssQ (6, 4), and aaP (2, 1) transitions, respectively. These values are comparable to pressure-broadening parameters for inversion transitions in the ground state, but they are significantly larger than corresponding values for inversion transitions in the v 2 = 1 state. This comparison is explained as being the result of a larger contribution from adiabatic reorientation and/or phase-changing collisions to the pressure-broadening of the infrared transitions than to the inversion transitions in either state. The limiting low-pressure values of the center frequencies of the transitions are reported and the pressure shifts in the center frequencies are given. Collisional-narrowing parameters for the three transitions are estimated.