Polarized single-site IR spectra of free-base porphyrin-d(4), -d(8), -d(12), and -N-15(4), isolated in xenon matrix and aligned by action of linearly polarized visible light, have been measured and compared with the previously obtained spectra of unlabeled porphyrin and porphyrin-d(2). There is much agreement but also a fair amount of disagreement with the symmetry assignments previously proposed in the literature. The results are compared with those of the empirical force fields proposed in the literature and with ab initio calculations at the 3-21G level of Hartree-Fock theory. It is concluded that experimental information on additional isotopomers and particularly, on gerade symmetry vibrations, will be needed before a reliable harmonic force field can be developed. The calculations require the introduction of electron correlation before the equilibrium ground state geometry and force constants can be calculated reliably enough to be useful.