Isothermal N-2 adsorption experiments were conducted with a multipromoted iron-based ammonia synthesis catalyst. From these measurements a simple model describing the adsorption process over the entire range of N coverages was developed. The model is consistent with both surface science experiments conducted with single-crystal iron surfaces and earlier experiments on promoted iron-based catalysts. The results from these experiments were well described, if it was assumed that Na dissociation occurred at only a few active sites, and that the rest of the adsorption sites were filled via diffusion from these sites. The number of active sites amounted to 4 mu mol/g, which is approximately 5 % of the total number of sites on the catalyst capable of binding nitrogen. The initial sticking coefficient, so, of dinitrogen on the active sites was found to be s(0) = 10(-5) exp(-(4 kJ/mol)/RT). (C) 2001 Academic Press.