Volumes of activation for the formation and homolysis of the transient complexes (hedta)FeIII-CO22- and (hedta)- FeIII-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 (M2+) and ligands (L) points out that (i) the reaction of MnLm with aliphatic radicals (R•) proceeds via an interchange ligand substitution mechanism, i.e. MnLm + 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 = CrIII, CoIII, NiIII and smaller for FeIII. The volume of activation for the reaction (hedta)FeIII-CO22- + CO2•- + 2H+ → FeII(hedta)(H2O)- + CO + CO2 was measured, and the results enable a tentative proposal for the nature of the transition state of this interesting reaction.