Abstract High resolution spectra of P-, As- and Sb-doped Si have allowed the determination of the “natural” width of the impurity lines for concentration in the range of 1014 at cm−3 and below. The width of the P-lines makes possible the observation of the 5p0 line, very close to the 4p± line. In the Sb- and As-doped samples, an effect similar to concentration broadening is observed. Results on P- and Bi-doped samples at lower resolution give evidence of the observation of the 3d0(P) line and of internal Stark effect in the Bi-doped sample.