In boron-doped silicon, optical absorption takes place through the excitation of bound holes from the ground state to excited states. This leads to a line spectrum. Because of a lack of sufficient resolution and a failure to make proper allowance for line distortion by the finite spectrometer slit width, previous authors gave a misleading picture of the low-temperature half-width, the temperature dependence of this half-width, and the onset of concentration broadening at low temperatures.New experimental data are presented and explained by introducing the mechanism of statistical Stark broadening due to ionized impurities, and by modifying Baltensperger's (1953) theory for concentration broadening. At low impurity concentration the width is attributed to phonon broadening (Barrie and Nishikawa 1963) and internal strains (Kohn 1957).