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The dimerization of bis(4-methylphenyl)acetylene and of 1-(4′-methylphenyl)-2-phenylacetylene yields in each case an active dilithium compound which reacts with dichlorodiphenylsilane and leads to the corresponding silacyclopentadiene.UV and IR spectra of these derivatives are presented. It has been demonstrated by NMR spectroscopy that the compound bearing two phenyl and two 4-methylphenyl substituents on the silacyclopentadiene ring contains several isomers, which result from the position of the aryl substituents. Three isomers are present according to NMR spectra and the proportion of each is determined.