Abstract The energy response of a full-energy-absorption neutron spectrometer using boron-loaded liquid scintillator BC-523 was examined for the neutron energy range between 1.2 and 14 MeV. Pulse height spectra measured with monoenergetic neutrons evidenced that they were characterized by either a single peak with a shoulder or by double peaks. A Monte Carlo simulation of the neutron behavior in the detector reproduced the spectra by considering the nonlinear light yield against recoil proton energy. Detection efficiencies ranging from 10% for 1.2 MeV to 0.6% for 14 MeV neutrons were obtained with a 12.7 cm diameter × 7.6 cm long scintillator. A high background rejection with a ratio of 2200 to 1 obtained without using any γ-shielding makes the present neutron spectrometer attractive for the application to low-level environmental neutron measurements.