The isotopic composition of the rare earth element neodymium (Nd) has the potential to serve as water-mass tracer, because it is naturally tagged by continental sources with distinct ages and lithologies. However, in order to understand the limitations of this approach we need to know more about the physical and biogeochemical processes controlling the distribution of Nd in the modern ocean. For example, Nd isotope ratios behave quasi-conservatively, while concentrations in the water column generally increase with depth, showing a broadly nutrient-like behaviour. We define this decoupling of Nd concentrations and isotopic compositions as the "Nd paradox". For the first time we model Nd concentrations and isotopic compositions simultaneously and address the hypothesis that the Nd paradox can be explained by a combination of lateral advection and reversible scavenging. We impose a reversible-scavenging model of Nd removal from the ocean on the ocean circulation fields from the MIT general circulation model using the transport matrix method. We conclude that reversible scavenging is an active and important component in the cycling of Nd in the ocean. In the absence of an adequate alternative explanation, reversible scavenging should be considered a necessary component in explaining the Nd paradox. © 2008 Elsevier B.V. All rights reserved.