Two hydrogen-related donors in anatase (Formula presented.) —interstitial hydrogen ((Formula presented.)) and hydrogen substituting for oxygen ((Formula presented.))—are addressed by means of ab initio density functional theory and vibrational mode spectroscopy. First-principles calculations and infrared (IR) absorption measurements reveal that (Formula presented.) forms an O-H bond lying in the basal plane of the crystal characterized by a stretch local vibrational mode (LVM) with a frequency of 3412 (Formula presented.). According to the calculations, the vibrational spectrum of the threefold-coordinated hydrogen in (Formula presented.) includes two stretch LVMs with the frequencies of 1014 and 1294 (Formula presented.). The theoretically predicted modes occur within the strong two-phonon absorption band of (Formula presented.), so the conclusions of theory cannot be verified by conventional IR absorption spectroscopy. In accordance with experimental findings, (Formula presented.) is shown to be unstable at room temperature, whereby formation of interstitial (Formula presented.) should be a preferential sink for hydrogen in defect-free anatase. It is also found that of (Formula presented.), (Formula presented.), and (Formula presented.), the latter is the most energetically favorable defect, which may account for the “hidden” hydrogen unavailable for the standard spectroscopic techniques.