The hydrolysis reactions of Np(VI) were investigated under an ambient atmosphere by attenuated total reflection Fourier transform infrared (ATR FT-IR) spectroscopy, NIR absorption spectroscopy, and speciation modeling applying the updated NEA thermodynamic database. For the first time, spectroscopic results of Np(VI) hydrolysis reactions are provided in the submillimolar concentration range and at pH values up to 5.3. The calculated speciation pattern and the results from FT-I R. spectroscopy are comparatively discussed with results obtained from the U(VI) system under identical conditions. For both actinides, the formation of similar species can be derived from infrared spectroscopic results at pH values <= 4, namely, the free cation AnO(2)(2+) (An = U, Np) and monomeric hydrolysis products. At higher pH, the infrared spectra evidence structurally different species contributing to the speciation of both actinides. At pH 5, the formation of a carbon ate-containing dimeric complex, that is, (NPO2)(2)CO3(OH)(3)(-), probably occurs during the hydrolysis reactions of neptunium, which is supported by the calculated speciation and results from NIR spectroscopy. For uranium, the presence of additional hydroxo complexes is assumed in this pH range. However, an unequivocal assignment of the spectral features to distinct species remains difficult. In particular, in the concentration range (0.5 mM) that constitutes the lower limit for the spectroscopic investigations of Np(VI) in the present work, monomeric and polymeric species obviously contribute to the U(VI) speciation considerably increasing the complexity of the spectral data.