Synchrotron-based X-ray absorption spectroscopy has been used to determine the chemical speciation of Np sorbed on Opalinus Clay (OPA, Mont Terri, Switzerland), a natural argillaceous rock revealing a micro-scale heterogeneity. Different sorption and diffusion samples with Np(V) were prepared for spatially resolved molecular-level investigations. Thin sections of OPA contacted with Np(V) solution under aerobic and anaerobic conditions as well as a diffusion sample were analysed spatially resolved. Micro-X-ray fluorescence (μ-XRF) mapping has been used to determine the elemental distributions of Np, Fe and Ca. Regions of high Np concentration were subsequently investigated by micro-X-ray absorption fine structure spectroscopy to determine the oxidation state of Np. Further, micro-X-ray diffraction (μ-XRD) was employed to gain knowledge about reactive crystalline mineral phases in the vicinity of Np enrichments. One thin section was also analysed by electron microprobe to determine the elemental distributions of the lighter elements (especially Si and Al), which represent the main elements of OPA. The results show that in most samples, Np spots with considerable amounts of Np(IV) could be found even when the experiments were carried out in air. In some cases, almost pure Np(IV) L(III)-edge X-ray absorption near-edge structure spectra were recorded. In the case of the anaerobic sample, the μ-XRF mapping showed a clear correlation between Np and Fe, indicating that the reduction of Np(V) is caused by an iron(II)-containing mineral which could be identified by μ-XRD as pyrite. These spatially resolved investigations were complemented by extended X-ray absorption fine structure measurements of powder samples from batch experiments under aerobic and anaerobic conditions to determine the structural parameters of the near-neighbour environment of sorbed Np.