Abstract
It is shown that electroabsorption measurements provide an elegant test to decide between the two most favored theories of Urbach's rule at excitonic absorption edges, namely the model of the field-ionized exciton by Dow and Redfield and the model of the momentarily trapped exciton by Sumi and Toyozawa. Experiments performed at the intrinsic Urbach tails of the ionic insulators CuCl and TlCl give support to the field-ionization model.