Bulk solid samples of various ratios of the cyclometalated arene ruthenium diastereomers (S)(Ru)- and (R)(Ru)-[(eta (6)-C6H6)Ru(C6H4-2-(R)-CH(Me)NMe2)PMe2Ph]+PF6- (3a/3b), of which the configurational stability at the metal center has been established by classical solution techniques, have been analyzed by the C-13 cross-polarization magic angle spinning (CP-MAS) and P-31 MAS NMR. The spectra obtained allowed us to detect both isomers and to estimate their respective proportions by P-31 spectra. This technique was applied to a bulk solid sample of the diastereomers (S)(Ru)- and (R)(Ru)-[(eta (6)-C6H6)Ru(C6H4-2-(R)-CH(Me)NMe2)NCMe]+PF6- (1a/1b), which were shown to be configurationally labile by classical solution experiments. Detection of isomer la only in the resulting C-13 CP-MAS NMR spectrum demonstrated that there has been epimerization of 1b to 1a during crystallization. thus confirming the configurational lability at the metal center.