Abstract The complexation of Np(IV) with gluconic acid (GLU) under alkaline conditions was investigated in the absence of Ca by carrying out a series of sorption experiments. The decrease of Np(IV) sorption on the sorbing material at increasing concentrations of GLU was interpreted as the formation of Np(IV)-GLU aqueous complexes. The modelling of experimental data according to the Schubert method [1] confirmed the formation of a complex with a Np : GLU ratio 1 : 1. The stoichiometry of the complex Np(OH)₄GLU⁻ was proposed based on the experimental observation that no proton exchange occurred during the course of the complexation reaction and that Np(OH)₄(aq) was the predominant hydrolysis product in the absence of GLU. A log ^* β _1,4,1 ⁰ = −2.92 ± 0.30 for the formation reaction Np⁴⁺ + 4H₂O + GLU⁻⇔Np(OH)₄GLU⁻ + 4H⁺ was calculated based on the conditional stability constants determined from sorption experiments and using the Np(IV) thermodynamic data selected in the NEA reviews [2]. Linear free energy relationships (LFER) confirmed that the stoichiometry and stability of the Np(IV)-GLU complex characterized in this work are consistent with data available for Th(IV)-, U(IV)- and Pu(IV)-GLU complexes.