Fourier-transform infrared spectroscopy in the frequency range 400-4000 cm -1 has been used to investigate the absorption of H 2 O and H 2 O:CO 2 complex isolated in solid argon. Thanks to the lowest temperature reached in our experiment, temperature effects and nuclear spin conversion studies allow us to propose a new assignment of the rovibrational lines in the bending band n 2 for the quasi-freely rotating H 2 O. An additional wide structure observed in this band shows two maxima around 1657.4 and 1661.3 cm -1 , with nuclear spin conversion of the high frequency part into the low frequency one. This structure is tentatively attributed to a rotation-translation coupling of the molecule in the cage. However, the equivalent effect is not observed in the vibrational stretching bands n 1 and n 3 . Finally, by double doping experiments with CO 2 , important new structures appear, leading us to unambiguously extract the frequencies of the lines of the H 2 O:CO 2 complex.