E.s.r. spectra have been recorded in solution for the substituted cyclopentadienyl radicals X3MC5H4., where X3M = Me3Si, Me2HSi, Me2ClSi, Me2FSi, Ph2ClSi, Cl3Si, Me3Ge, Bu3Ge, Ph2ClGe, and Me3Sn. The spectra can be interpreted in terms of perturbation of the orbitals ψA and ψS which are degenerate in the C5H5. radical. This allows the substituents to be placed in the following sequence regarding their electronic interactions, compared with that of hydrogen, with the ψS MO of the cyclopentadienyl π-system: electron repelling, Me3C ≫ Bu3Ge ∼ Me3Ge > H; electron attracting, H < Me3Sn < Me3Si < Me2HSi < Me2ClSi < Ph2ClGe < Me 2FSi < Ph2ClSi < Cl3Si.