Volumes of activation for the formation and homolysis of the transient complexes (hedta)Fe-III-CO22- and (hedta)Fe-III- CH3-(HOCH2CH2N(CH2CO2-)CH2CH2N(CH2CO2-)(2) = hedta) were determined using high-pressure pulse-radiolysis techniques. A comparison of the results with those for analogous complexes with other central transition-metal cations (Mn+) and ligands (L) points out that (i) the reaction of (MLm)-L-n with aliphatic radicals (R-.) proceeds via an interchange ligand substitution mechanism, i.e. (MLm)-L-n + R-. --> Lm-1Mn+1-R + L, (ii) the homolysis of the metal-carbon bonds naturally follows the same mechanism, and (iii) the volume of activation for the homolysis reaction depends strongly on the nature of the central cation, i.e. larger for Mn+1 = Cr-III, Co-III, Ni-III and smaller for Fe-III. The volume of activation for the reaction (hedta)Fe-III-CO22- + CO2.- + 2H(+) --> Fe-II(hedta)(H2O)(-) + CO + CO2 was measured, and the results enable a tentative proposal for the nature of the transition state of this interesting reaction.