The deposition of thin Ce films on Si(111) and subsequent or simultaneous annealing leads to the formation of various structurally, electronically, and chemically different CeSix phases, as revealed by scanning-tunneling microscopy, low-energy electron diffraction, and photoelectron spectroscopy. Several phases can coexist on the same sample, and with increasing annealing temperatures larger homogeneous domains are observed. This behavior is assigned to a mismatch of the lattices of the Si-rich CeSix bulk compounds with that of the Si(111) substrate surface, resulting at temperatures above 700 degrees C in the formation of a thin film of hexagonal CeSi1.67, which does not exist as a bulk compound.