A systematic investigation of the uv-light-induced photodissociation of substitutional O${\mathrm{H}}^{\ensuremath{-}}$ defects in KCl has been performed in the whole temperature range between 4 and 300 K. The primary and secondary processes and their reaction products (${\mathrm{O}}^{\ensuremath{-}}$, ${U}_{2}$, $U$, and H centers) are well understood and accounted for up to the point when the H center (${\mathrm{Cl}}_{2}^{\ensuremath{-}}$ crowdion) becomes thermally mobile and reactive, leading to an unexplained loss of hydrogen defects. A detailed study of the related local-mode spectra in the infrared identifies this missing hydrogen by a new local-mode absorption band (tested in KCl, KBr, KI, and RbBr). Studies of the formation-kinetics of this new ${{\mathrm{H}}_{x}}^{\ensuremath{-}}$ defect show that it is produced by the reaction of mobile ${\mathrm{Cl}}_{2}^{\ensuremath{-}}$ crowdions with ${\mathrm{H}}^{\ensuremath{-}}$ defects. The same ${{\mathrm{H}}_{x}}^{\ensuremath{-}}$ centers can be produced by low-temperature x irradiation of KCl: ${\mathrm{H}}^{\ensuremath{-}}$ crystals. From these studies it becomes evident that the ${{\mathrm{H}}_{x}}^{\ensuremath{-}}$ defect is composed from the same electronic and ionic ingredients as the ${U}_{2}$ center, thus constituting an alternative stable configuration for this defect with completely different electronic and ionic structure. Among the possible structural models which are discussed, a mixed ${(\mathrm{H}\ensuremath{-}\mathrm{C}\mathrm{l})}^{\ensuremath{-}}$ dumbbell model appears to be the most likely one.