Thermodynamic stereocontrol of the (hexamethyldisilazide)magnesium enolates of propiophenone in THF is reported. The overall stereoselectivity proves to be very sensitive to concentration, since dimeric species with bridging enolates show no stereoselectivity while monomeric enolates show a very strong thermodynamic preference for the Z enolate. Kinetically, interconversion among aggregates is remarkably slow, whereas stereoisomerization of the monomer, even in the absence of a proton source such as ketone or amine, is remarkably fast, Furthermore, stereoisomerization takes place in the absence of a proton source or excess ketone. These observations contrast with accepted views of these fundamentally important processes and have implications for understanding the identity and reactivity of metal enolates.