Abstract The reaction of lithium atoms with water has been studied using infrared matrix isolation spectroscopy. H/D and 6 Li/ 7 Li isotopic substitutions and irradiation effects in both visible and IR regions allow us to identify two kinds of complexes: after deposition a 1-1 species, of charge transfer type, characterized by rather small frequency shifts but strong intensity changes of the water bands: after irradiation either in the near IR or in the visible formation of the LiOH molecule identified by two absorptions below 1000 cm −1 : theLiOH stretching mode around 900 cm −1 and the bending mode at 257 cm −1 . Vibrational calculations on both frequencies and intensities of these complexes allow us to analyse the charge transfer effect in the 1-1 complex and to conclude that the structure of the LiOH molecule is linear.