The study deals with the complex-forming properties of a fibrous complexing agent (complexite) NAG with respect to nickel(II) ions in a mixed solvent water-1,4-dioxane. Spectral tests indicate that the absorption of nickel(II) ions by the NAG fibrous complexer occurs according to the mechanism of complexation. The factors affecting the complex formation of the NAG complexite with nickel(II) ions in water-1,4-dioxane mixtures are established, and the stability constants of hydroxamic and amidoxime groups are calculated depending on the pH values of the medium and the composition of the solvent. The influence of the solvation parameter on the composition and stability of the complexes under study with nickel(II) ions has been established. The solvation characteristics of the NAG complexite and complexes with nickel(II) ions depend on the nature of the mixed solvent in which the complexation reaction takes place. The solvation effects in the water-1,4-dioxane mixture with a molar fraction of dioxane of 0.00–0.17 level the stability of the resulting complexes. It is shown that the pH range of 6.2–3.8 mixed solvent creates the conditions for the formation of a wider composition and structure of the range of coordination units with different proportions of their content in the polymer. Apart from the hydroxamic, deprotonated amidoxime groups take part in the complexation. The preferred fixation of functional groups on the surface of NAG fibres leads to a high local concentration of the reaction centres and enhances the cooperativeness of the process, facilitating easy orientation of the groups during the formation of mixed-ligand coordination nodes. The stability constants of the deprotonated amidoxime groups of the the NAG complexite with nickel(II) ions are estimated. The study has proved dependence on the composition of the solvent