The main purpose of this paper is to analyze recent nitrogen adsorption and desorption studies of iron-based ammonia synthesis catalysts by Langmuir-type models, to some extent with a linear coverage dependence of the activation energies included. The results are applied to set up a simple microkinetic model for the ammonia synthesis reaction, where due care has been taken to choose the kinetic parameters for the relevant range of coverages. This model is self-consistent in that the predicted coverages are within the range used to determine the input parameters, and the agreement between calculations and experiments is much better than for models based on single-crystal studies. A consequence of the analysis is that chemisorbed atomic nitrogen (N-*) is much more weakly bound at high coverages than is to be expected from low-coverage single-crystal studies. The coverage of N-* is close to 0.5 under synthesis conditions, a result that is in agreement with earlier considerations. The experimentally observed differences in the activity, as well as the pressure dependence of the reaction rate, working with catalysts with and without potassium promoter, can be at least partly explained by a destabilization effect of potassium on N-* or on the hydrogenated species NH-*. The main conclusion is that it is necessary to take surface heterogeneity or adsorbate-adsorbate interactions into account to obtain a model that can compete with the Temkin-type models.