Abstract The structure of 1(NO 2 )—dioxane 2:1 solvate single crystal has been determined by direct methods and refined by analysis of the three-dimensional Mo Kα intensity data. The unit cell is monoclinic, space group P2 1 /c with a = 8·960(3), b = 6·386(2), c = 30·141(11) Å, β = 108·56(2)°. Least-squares refinement of all positional and thermal parameters (hydrogen atoms isotropic) with 1458 independent reflections out of 2817 gave a final R-index of 0·069. The dye molecule is almost planar and of trans configuration. The nitro group, however, is significantly twisted out of the molecular best plane. Six-membered chelate ring structures are formed by intramolecular NH···N bonds while intermolecular NH···O interactions give rise to complex formation involving two dye and one dioxane molecules in a centrosymmetrical arrangement. Two such complexes are accommodated in the unit cell. Based on the observed molecular geometry and quantum chemical (SCF—MO) calculation of bond dimensions a strong predominance of the azo tautomer was concluded for 1(NO 2 ). This assignment is further supported by empirical correlations, using CN and NN bond distances from a great variety of reported azo and hydrazone structures which are compiled here for the first time. The mobile hydrogen is attached to the amine nitrogen [R(NH) = 1·04(5) Å ] thereby excluding a major participation of the hydrazone form. These findings compare well with those for solutions and are in accordance with previous quantum chemical predictions.