An infrared band system consisting of a number of vibrational bands, all belonging to a new electronic transition (A^{\prime 2} \mathrm{II}{1}-X^{2} \Sigma^{+})of the AIS radical, has been recorded with Fourier transform techniques. The bands are analogous to the bands of the (A^{2} \Pi{1}-X^{2} \Sigma^{+})transition of (A 1 O) at (2 \mu \mathrm{m}). In total, 28 vibrational bands consisting of 23000 rotational lines have been included in the rotational analysis. As in the case of (\mathrm{AIO}), the spin splittings of the (X^{2} \Sigma^{+})state show clearly noticeable nuclear hyperfine-induced effects. Numerous local perturbations affect the (A^{\prime 2} \Pi_{i}) state, all shown to be caused by vibrational levels of the (X^{2} \Sigma^{+})state. A fit of the (X^{2} \Sigma^{+})state has been performed both with and without hyperfine parameters. Attempts to perform a global deperturbation involving the (X^{2} \mathbb{\Xi}^{+}(v=0-13)) and (A^{\prime 2} \Pi_{i}(v=0-8)) levels have been made. The structure of the (X^{2} \Sigma^{+})state has been discussed in the light of results from the (A^{2} \Pi_{i}-X^{2} \Sigma^{+})system of (A I O), and a model has been proposed suggesting that the spin splittings of the (X^{2} \Sigma^{+}(v=0)) levels in both molecules are mainly due to nuclear hyperfine interaction and second-order spin-orbit effects due to the high-lying regular ({ }^{2} \mathrm{II}) state ( (C^{2} \mathrm{II}) in (\mathrm{AlO}) and (B^{2} \mathrm{II}) in (\mathrm{AIS}) ), while the influence of the low-lying inverted ({ }^{2} \Pi) state increases dramatically upon increasing (v) ". These counteracting trends result in a seemingly pure ( (\left.b_{s S}\right)) coupling for (X^{2} \Sigma^{+}(v=1)) in AIO and for the (N<60) region of (X^{2} \Sigma^{+}(v=2)) in AIS. Support for this interpretation has been obtained by comparing synthetically generated hyperfine spectra with the observed line profiles in transitions to the (X^{2} \Sigma^{+}(v) (=0,1,2) ) levels of AIS and AIO. (c) 1994 Academic Press. Inc.