Using measured isotope shifts for the 607 cm-1 local vibrational mode, LVM, of substitutional carbon, Cs, we demonstrate that isotope disorder contributes ∼0.5 cm-1 to the width of that LVM in natural silicon. The width of the LVM of Cs also depends on its energy relative to the density of two-phonon states, and increases along the sequence 13C in natural silicon, 12C in natural silicon and 12C in 30Si. Other LVMs show larger isotope effects, and so discrete structure rather than line broadening. In the case of zero-phonon lines, we take the 3942 cm-1 line as a potentially favourable example. We estimate that the isotope disorder contributes only 0.09 cm-1 to the linewidth in a natural silicon sample, a contribution that is negligible compared to typical strain-broadening effects, but would be a lower limit to the linewidth in high-purity natural silicon.