We report the observation of infrared (IR) vibrational bands related to individual thermal double donors (TDs) in Ge crystals enriched with either 16Oor18O isotopes of oxygen. The TDs were generated by heating the oxygen-doped Ge at 300°C and 350°C. IR absorption spectra were measured at room temperature (RT) and 10K (LT). In the RT spectra two broad bands at about 600 and 780cm−1 are found to develop upon the TD generation. In the LT spectra a splitting of the bands into series of rather sharp bands (up to 9 resolved lines) related to a double-donor (DD) configuration of individual TDs (TD1–TD9) is observed. A manifestation of bistability of the first four TD species (TD1–TD4) is observed in absorption spectra measured at 10K after different cooling conditions. The pairs of lines due to bistable TDs in the DD configuration are detected after cooling under the band-gap illumination. They are found to transform to the sets of three lines after cooling in the dark. These triplets are assigned to the local vibrational modes (LVMs) of a neutral (X) configuration of bistable TDs. Oxygen isotopic shifts of LVMs due to TDs are determined and compared with those for vibrational modes of interstitial oxygen in Ge.