[en] Transmuting ²³⁷Np is among the fundamental tasks of minor actinide transmutation. This may be realized by reactor techniques when small admixtures of ²³⁷Np are put in solid fuels of breeders and by the latest technologies that involve continuous ²³⁷Np burnup in a heavy water solution. This solution can be recirculated in subcritical blankets of electronuclear heavy water or molten salt blanket facilities with high energy particle nucleus collisions providing the neutron source. Here, experimental determination of the resonance neutron absorption in a heavy water solution of 2.57 g/ell ²³⁷Np is presented. Experimental samples were irradiated in the MAKET critical plant. Prior to irradiation, ²³⁷Np was radiochemically purified to remove ²³³Pa. The experimental samples were prepared by evaporating two aliquot samples taken from the irradiated solution. The nuclide composition of the samples was analyzed using a GC-2518 Ge detector, a 1510 module, and an S-100 plate to emulate a multichannel analyzer in an IBM personal computer. The spectrometric resolution was 1.8 keV at the 1332-keV line. The gamma spectra were processed by the ASPRO code. The neutron flux was measured by two methods using Al + ⁵⁵MN + ^natCu + ¹⁹⁷Au + ^natLu alloy foils