The (\nu_{2}+\nu_{3}-\nu_{2}) hot band of (\mathrm{NH}{3}^{+}\left(\dot{X}^{2} A{2}^{\prime \prime}\right)) has been observed on the absorption spectrum recorded by using a difference frequency laser spectrometer combined with the velocity modulation technique in the region between 3500 and (2900 \mathrm{~cm}^{-1}). The molecular ions were produced in the positive column of a glow discharge with a gas mixture of (\mathrm{N}{2} / \mathrm{H}{2} / \mathrm{He}) in a ratio around (1 / 1 / 100). From the observed spectrum, the band origin was determined (\left(\nu_{0}=3371.5607(21) \mathrm{cm}^{-1}\right)) and the rotational structure characterized up to (N, K=10, N-4). A symmetric rotor Hamiltonian with (A_{1}-A_{2}) splittings and (/) resonance was used for the analysis and the spin-rotation constants were determined. The values obtained for the centrifugal distortion constants of both states do not satisfy the planarity relations. This fact is interpreted as an evidence for the presence of an (x-y) Coriolis interaction between the (\nu_{2}) and the (\nu_{4}) states, in agreement with theoretical predictions by Kraemer and Spirko (J. Mol. Spectresc. 153, 276-284, 1992). The lack of experimental observation on (\nu_{4}) prevented a full treatment of the perturbation. (c. 1994 Academic Press. Inc.