In this work, phosphorus donors in diamond have been investigated by means of Fourier Transform Infra-Red (FTIR) transmission spectroscopy on a series of fully-characterized homoepitaxial layers. Low-temperature FTIR absorption spectra have been obtained for concentrations of donors in the range ND = 3.7 × 1016– 3.5 × 1018 cm− 3 and compensating acceptor impurities in the range NA = 2.3 × 1016– 3.0 × 1018 cm− 3. The absorption spectra is shown to exhibit two peaks at 4220 cm− 1 and 4540 cm− 1 corresponding to electronic transitions from the ground state to the 2p0 and 2p± excited states, respectively, of phosphorus donors in the neutral state of charge. The intensity of the most intense peak at 4540 cm− 1 is found proportional to ND-NA over two decades. The absorption cross-section of the corresponding transition is deduced providing a calibration law for the determination of ND-NA in phosphorus-doped diamond. Furthermore, the linewidth of the peak exhibits a linear variation with the compensating acceptor concentration, interpreted as the effect of the distribution of ionized impurities. The use of the non-destructive and contact-less FTIR optical method is finally discussed to determine the compensation ratio in phosphorus-doped diamond.