Abstract The ranges of epitaxial deposition have been explored for Ag evaporated simultaneously under high vacuum onto three NaCl substrate surfaces designated (111), (100) E and (100) c. The first substrate consisted of a thin (111)-oriented monocrystalline NaCl film deposited onto air-cleaved mica. The substrate designated (100) E consisted of a thin (100)-oriented monocrystalline NaCl film evaporated onto air-cleaved NaCl, and that designated (100) c consisted of air- cleaved NaCl. The occurrence of monocrystalline and polycrystalline films and their microstructure were determined by transmission electron microscopy and diffraction as a function of deposition rare R and substrate temperature T. The results are presented on 1n R versus 1 T plots. Activation energies for the transition from a low temperature polycrystalline region to a higher temperature monocrystalline region were determined as approximately 0.8 and 1.0 eV respectively for (111) and (100) E NaCl substrates. No such transition was found in the case of (100) c substrates. In the case of (100) E substrates a steep high temperature demarcation line (activation energy 3.9 eV) was found to separate the monocrystalline region from a high temperature polycrystalline region. The latter region was identified with the coalescence stage of film growth. These results agree with earlier reports suggesting that the coalescence stage rather than the nucleation stage is pre-eminent in determining the occurrence of epitaxy.