  It has recently been suggested that the power spectrum of redshifted 21cm
fluctuations could be used to measure the scale of baryonic acoustic
oscillations (BAOs) during the reionisation era. The resulting measurements are
potentially as precise as those offered by the next generation of galaxy
redshift surveys at lower redshift. However unlike galaxy redshift surveys,
which in the linear regime are subject to a scale independent galaxy bias, the
growth of ionised regions during reionisation is thought to introduce a
strongly scale dependent relationship between the 21cm and mass power spectra.
We use a semi-numerical model for reionisation to assess the impact of ionised
regions on the precision and accuracy with which the BAO scale could be
measured using redshifted 21cm observations. For a model in which reionisation
is completed at z~6, we find that the constraints on the BAO scale are not
systematically biased at z > 6.5. In this scenario, and assuming the
sensitivity attainable with a low-frequency array comprising 10 times the
collecting area of the Murchison Widefield Array, the BAO scale could be
measured to within 1.5 per cent in the range 6.5 < z < 7.5.
