We study the growth of the c(2 x 2) structure of Pb on Ni(001) with hi
gh-resolution x-ray diffraction. By monitoring the adsorbate-substrate
and the adsorbate-adsorbate interference diffraction peaks, we are ab
le to determine the registry and growth process of the first overlayer
. Pb atoms change from hollow site to non-hollow site growth as covera
ge increases. Results at higher substrate temperatures indicate a diff
erent growth process at low coverage. We propose a model of adsorbate
aggregation at sites in line with Ni atoms around pinning centres to a
ccount for the observed results.