The interaction of Np(v) with borate was investigated in 0.1-5.0 M NaCl and 0.25-4.5 M MgCl2 solutions with 7.2 ≤ pHm ≤ 10.0 (pHm = -log[H+]) and 0.004 M ≤ [B]tot ≤ 0.16 M. Experiments were performed under an Ar-atmosphere at T = (22 ± 2) °C using a combination of under- and oversaturation solubility experiments, NIR spectroscopy, and extensive solid phase characterization. A bathochromic shift (≈5 nm) in the Np(v) band at λ = 980 nm indicates the formation of weak Np(v)-borate complexes under mildly alkaline pHm-conditions. The identification of an isosbestic point supports the formation of a single Np(v)-borate species in dilute MgCl2 systems, whereas a more complex aqueous speciation (eventually involving the formation of several Np(v)-borate species) is observed in concentrated MgCl2 solutions. The solubility of freshly prepared NpO2OH(am) remained largely unaltered in NaCl and MgCl2 solutions with [B]tot = 0.04 M within the timeframe of this study (t ≤ 300 days). At [B]tot = 0.16 M, a kinetically hindered but very significant drop in the solubility of Np(v) (3-4 log10-units, compared to borate-free systems) was observed in NaCl and dilute MgCl2 solutions with pHm ≤ 9. The drop in the solubility was accompanied by a clear change in the colour of the solid phase (from green to white-greyish). XRD and TEM analyses showed that the amorphous NpO2OH(am) "starting material" transformed into crystalline solid phases with similar XRD patterns in NaCl and MgCl2 systems. XPS, SEM-EDS and EXAFS further indicated that borate and Na/Mg participate stoichiometrically in the formation of such solid phases. Additional undersaturation solubility experiments using the newly formed Na-Np(v)-borate(cr) and Mg-Np(v)-borate(cr) compounds further confirmed the low solubility ([Np(v)]aq ≈ 10-6-10-7 M) of such solid phases in mildly alkaline pHm-conditions. The formation of these solid phases represents a previously unreported retention mechanism for the highly mobile Np(v) under boundary conditions (pHm, [B]tot, ionic strength) of relevance to certain repository concepts for nuclear waste disposal.