The disposal of high level nuclear waste in deep geological formations poses major scientific and social challenges to be met in the next decades. Over geological time spans it cannot be excluded that groundwater will infiltrate into a waste repository. It needs to be considered that permeating groundwater corrodes waste canisters and mobilizes radionuclides from the waste. Demonstrating the repository safety requires a sound understanding of the migration and retention behavior of radionuclides in the geosphere. In this experimental study sorption reactions of neptunyl(V) with the mineral phase calcite have been investigated. The calcite surface is investigated by means of zetapotential and surface diffraction measurements. The surface adsorption of neptunyl(V) at calcite is investigated over a large range of Np concentrations and pH. Upon coprecipitation neptunyl(V) is readily incorporated into the calcite structure. The structures of an adsorption complex and the incorporation species are characterized by EXAFS spectroscopy. The reactivity of the calcite surface makes calcite a potentially important sink for neptunyl(V) in the geosphere.