The planar and nonplanar conformers of metal derivatives of 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) and 5-nitro-2,3,7,8,12,13,17,18-octaethylporphyrin (NO2-OEP) are investigated using resonance Raman spectroscopy. The structural heterogeneity is assessed by analysis of the line shapes of the structure-sensitive Raman lines. First, heterogeneity in the conformation of the macrocycle has been detected in solutions of the nickel and cobalt derivatives of OEP, that is, both planar and nonplanar conformers are found to coexist at room temperature for these metal porphyrins but not for the Cu and Zn derivatives. The latter metals expand the porphryin core, shifting the equilibrium entirely to the planar conformer. Second, we find that substitution with a single NO2 group at one of the methine-bridge carbons shifts this planar-nonplanar equilibrium substantially toward the nonplanar conformer. Thus, both crowding of the peripheral substituents and contracting of the porphyrin core (Ni(II) < Co(II) < Cu(II) < Zn(II)) displace the equilibrium toward the nonplanar conformer. Finally, the frequencies of several Raman lines correlate with structural parameters such as core size (obtained either from molecular mechanics calculations or from X-ray crystallographic studies). The calculations predict and the marker line frequencies verify that a small expansion of the core results from the steric repulsion between the nitro and the ethyl groups. Core size dependence of the intensities and frequencies of the NO2 stretching vibrations suggests that the NO2 stretches are coupled to nearby vibrational modes of the porphyrin macrocycle.