The nucleation and initial growth model developed by Routledge and Stowell has been used to analyse the experimental data obtained by Robins and co-workers for Au and Ag deposited onto alkali halide substrates. It is shown that, although nucleation parameters can be readily deduced for these systems, the agreement between theory and experiment is not wholly satisfactory. The discrepancies may be partly removed by modifying the original calculations to account for dimer mobility but cluster size distribution effects are significant and need to be incorporated into the theoretical model in order to obtain a more detailed description of the experimental results.