Here we present and analyse the behaviour of four different parameterizations of the preferential uptake of NH4+ over NO32 -. The first three (Wroblewski-Fasham, Spitz, Hurtt and Armstrong) are commonly used in current marine ecosystem models. However, they show either formulational inconsistencies (e.g. Wroblewski-Fasham), unrealistic features (e.g. Spitz) or structural rigidity (e.g. Hurtt and Armstrong). We therefore suggest a newer parameterization by slightly modifying the Wroblewski-Fasham formulation that easily overcome these deficiencies and could be used in the next generation of marine ecosystem models.