[en] Thermal annealing of electron-irradiated Czochralski (Cz) silicon containing a high concentration of phosphorous (∼1.10¹⁷ P-atoms/cm³) has been performed. Low temperature (∼10 K) infrared (IR) absorption measurements were used for the analysis. The dominating IR-bands directly after irradiation appear at 835 and 885 cm^-1 and are attributed to a vacancy-oxygen complex (VO) in a neutral and a negative charge state, respectively. During a subsequent isochronal annealing the VO-pairs start to diasappear already at temperatures of about 230^oC, while in more modestly doped Cz-silicon the VO-pairs are stable up to ∼300^oC. Furthermore, a new (not previously reported) band at 916 cm^-1 emerges during the annealing sequence. (author) 4 figs., 1 tab., 12 refs