The feasibility of performing simultaneous RHEED–mass‐spectrometric‐molecular‐beam experiments is demonstrated with a study of the nucleation of Cd on a contaminated Ge(111) surface. The crossed‐beam experimental geometry and the scheme for determining absolute adatom density is described. The nucleation mechanism is found to have the appearance of a two‐dimensional gas‐solid phase transformation. The critical adatom density was found to be dependent on the level of contamination. The onset of clustering was found to correlate well with the MSMB spectra and clusters approaching critical dimensions were observed.