Abstract The infrared vibrational absorption attributed to thermal donors (TDs) in Ge has been investigated. Oxygen-doped Ge was annealed at 350°C and the spectrum was recorded after different annealing times. Absorption bands develop mainly around 600 and 780 cm −1, in close correspondence with the increasing TD concentration and the loss of interstitial oxygen (O i). At low temperature the bands are resolved into a number of components which appear sequentially and are assigned to the double donor configuration of TD2–TD9. The assignments are confirmed by the bistable properties of TD2–TD4 revealed in the vibrational spectrum. The low-energy configuration gives rise to three new bands in the same range. The absorption is interpreted as due to vibrational modes of oxygen incorporated in the TDs. The number of oxygen atoms involved is discussed taking account of the evolution of the O i and TD concentrations and of the integrated absorption.