Foil irradiation experiments were carried out at LFR (ECN. Petten) and WWR-SM (KFKI, Budapest). The effect of boron covers on the activation and fission detector response regions has been investigated. Fluence rate perturbation due to the applied B⁴C box in graphite or water was determined by means of transport calculations. Neutron spectrum adjustments were performed by the SAND-II and STAY'SL codes. The B⁴C cover shifts the response regions of the resonance reactions from the thermal region to the 0.1 keV to 1.0 MeV interval. Where boron-covered reactions are used in a least squares spectrum adjustment procedure, the uncertainty of the spectrum is reduced significantly in the intermediate energy region.