The ortho-methyl groups on meso-phenyl substituents were shown to strongly inhibit CO oxidation by Rh tetraphenylporphyrins. When both of the ortho-positions were occupied by methyl groups, CO oxidation was dramatically inhibited. The CO adduct was characterized by spectroscopy. O-2 reduction by Rh tetraphenylporphyrins with ortho-methyl groups on meso phenyl substituents was also examined. The effects of ortho-methyl groups on CO oxidation are discussed in terms of steric hindrance. (C) 2012 Elsevier B.V. All rights reserved.