PACS 71.55.Cn, 78.20.Ci, 78.30.AmThe IR absorption and photoconductivity spectra of a double donor (TDD) family with ionization energies less than 70 meV in silicon were investigated using Fourier transform spectroscopy with a resolution down to 0.25 cm -1 . The values of line frequencies, widths, splitting and absorption cross sections for transitions from the ground to p-states (including high-excited states) were obtained. Determination of absorption cross sections was made using Hall data and concerted changes of boron and TDDi concentrations caused by neutralization of impurities under band gap illumination.Introduction Oxygen-containing thermal donors in Si along with B and P are widespread impurityrelated centers in Cz-Si. Investigations of the structure, energy spectrum and formation kinetics of various families of thermal donors have been carried out by various methods of IR spectroscopy [1 −3]. At the same, time only one work is known in which the absorption cross sections for transitions 1S → 2P 0 at a maximum of the lines for six thermal double donors have been determined in experiments with a low resolution [4]. In this report the results of detailed studies of IR absorption and photoconductivity spectra of TDDs with ionization energies less than 70 meV are given.