Abstract The dimethylmetallocenes Cp 2 Zr (13 CH 3) 2,(Me 5 Cp) 2 Zr (13 CH 3) 2,(indenyl) 2 Zr (13 CH 3) 2, Me 2 Si (C 5 H 4) 2 Zr (13 CH 3) 2 and Cp 2 Hf (13 CH 3) 2 undergo degenerate methyl exchange with the methyl-acceptor materials Me 6 Al 2,(MeAlO) x and MeAl (2, 6-di-t-butyl-4-methylphenoxy) 2. Free energies, enthalpies and entropies of activation for these processes have been obtained in two solvents by 13 C NMR. ...