  We investigate the feasibility of detecting 21cm absorption features in the
afterglow spectra of high redshift long Gamma Ray Bursts (GRBs). This is done
employing simulations of cosmic reionization, together with the instrumental
characteristics of the LOw Frequency ARray (LOFAR). We find that absorption
features could be marginally (with a S/N larger than a few) detected by LOFAR
at z>7 if the GRB originated from PopIII stars, while the detection would be
easier if the noise were reduced by one order of magnitude, i.e. similar to
what is expected for the first phase of the Square Kilometer Array (SKA1-low).
On the other hand, more standard GRBs are too dim to be detected even with ten
times the sensitivity of SKA1-low, and only in the most optimistic case can a
S/N larger than a few be reached at z>9.
