AbstractIt is a common practice in ENDF/B to represent neutron cross sections in the unresolved resonance region by specifying the average values and distribution laws of resonance parameters. This formalism allows the calculation of resonance self-shielding and of the variation of resonance selfshielding with temperature, two important reactor parameters. For many applications it is necessary to estimate the uncertainties in these model average resonance parameters. A possible approach to derive such uncertainties is described, using as an example the ENDF/B-V representation of238U in the range from 4 to 45 keV.