The equilibrium between ammonia (NH3) and ammonium (NH4+) in aqueous solution is a function of temperature, pH and the ionic strength of the solution. Here we reveal a 30-year-old error in published work on the thermodynamics of ammonium dissociation in seawater, which has propagated throughout the literature. The work in question [K. H. Khoo, C. H. Culberson, R. G. Bates, J. Solution Chem. 1977, 6, 281] presents an incorrect expression for the variation of the acid dissociation coefficient (Ka) of ammonium with temperature and ionic strength. We detail the error and reveal that it can lead to as much as a 500% overestimation in calculated NH3 concentration under environmental conditions. This finding is highly relevant, particularly for studies of ammonia toxicity and air–sea ammonia exchange. In addition, we recommend two expressions that better reproduce previous experimental work: (i) taken from the work of Johansson and Wedborg, and (ii) our own derivation using the dataset of Khoo et al.