The quantum yields (ϕ) for the photochemical arene release from [Cp*M(η6-arene)] + complexes (Cp* = η5-pentamethylcyclo-pentadienyl) have been measured for M = Fe(II) and Ru(II) in acetonitrile solutions at room temperature. (Fe: arene = toluene (ϕ = 0.007), hexamethylbenzene (ϕ = 0.00002). Ru: arene = benzene (ϕ = 0.19), mesitylene (ϕ = 0.025), hexamethylbenzene (ϕ= 0.0019).) For both M = Fe and Ru, permethylation of the cyclopentadienyl ligand decreases the arene release quantum yields as compared to those of the analogous cyclopentadienyl compounds. For M = Fe, the decreases are factors of 113 and 20 500, respectively, for arene = toluene and hexamethylbenzene. The very large decrease observed for the (pentamethylcyclo-pentadienyl)(hexamethylbenzene)iron complex is consistent with an important steric blocking effect of the five pentamethyl-cyclopentadienyl methyl groups. For the three (pentamethylcyclopentadienyl)ruthenium compounds studied, decreases are more modest relative to the cyclopentadienyl compounds (1.8, 3.4, and 7.4 for M = Ru, where arene = benzene, mesitylene, and hexamethylbenzene). In this case, a linear correlation persists between log (ϕ/(1-ϕ)) and σp, the Hammett parameter for methyl substituents. The linearity of the log (ϕ/(1-ϕ)) vs. σp plot indicates that steric effects are not present to hinder the participation of acetonitrile in the transition state of the highly methylated Ru complexes studied. The increase in the Hammett ρ parameter from +1.38 to +2.05 upon cyclopentadienyl methylation of the Ru complexes suggests a slight increase in negative charge occurs at the arene in the arene release transition state of the pentamethylcyclopentadienyl complexes relative to the cyclopentadienyl complexes.