The growth of epitaxial layers of hexagonal theta-nickel-silicide on Si(100) and Si(111) substrates is reported. They form at 370 degrees C on Si(100) and 360 degrees C on Si(111), from codeposited Ni/Si mixtures, containing 37 to 42 atom % Si and the equivalent of a 50 nm Ni layer. These codeposited layers model the Ni/Si mixing layer at the interface in sputter-deposited films. The occurrence and stability at room temperature conflict with the phase diagram for bulk Ni/Si. Congruent crystallization is shown to initiate the growth of this metastable phase. (C) 2008 The Electrochemical Society.