The structure of the previously synthesized 1,2-dihydro-3-(2-hydroxyphenyl)-4-nitropyrazol-5(3H)-one (I) was studied by 1H and 13C NMR, IR, and electronic spectra. It was established that the NMR spectra of compound (I) do not contain signals for the H atom at position 4 and the C atoms of the carbonyl groups; in the crystalline state and in solutions the IR spectra do not contain vc=o bands, while the electronic spectra do not contain absorption in the region of 250 nm. On the basis of the foregoing it is concluded that compound (I) exists in the OH form. The distribution of π-electron density in the molecule of (I) and the nature of the bands in its electronic absorption spectrum were studied by quantum-chemical methods in the PPP approximation. © 1991 Plenum Publishing Corporation.