The rotational strength (R) of the Soret transition in sperm-whale myoglobin (SW Mb), the hemoglobin from Chironomus thummi thummi (CTT Hb), and human hemoglobin (hHb) has been calculated using 20 high-resolution (< 1:5 Å) crystal structures. The intrinsic rotational strength due to heme non-planarity was calculated using π-electron theory and time-dependent density functional theory (TDDFT). Calculations on model protoporphyrins with a planar nucleus and with various torsional angles for the 2- and 4-vinyl substituents showed maximum R of ±0.70 Debye-Bohr magneton (1 DBM = 0.9273 · 10 -38 cgs units). Viewing the heme so that the 2- and 4-vinyls are in a counterclockwise relationship, if a vinyl points toward the viewer, it contributes positively to R. Calculations of the intrinsic R for explicit heme geometries of SW Mb, CTT Hb, and hHb gave averages of 0.40±0.09, ±0:44±0.04, and +0.32±0.11 DBM, respectively. Coupling of the Soret transition with aromatic side-chain and peptide backbone transitions was also considered. For SW Mb, the magnitudes of the contributions decreased in the order R int > R aro > R pep . For CTT Hb and hHB, the orders were, respectively, R int > R pep > R aro and R int > R aro ≈ R pep . Human Hb ɑ chains showed the same trend as CTT Hb. Only in the hHb β chains did R aro predominate, with the order R aro > R int > R pep . The total predicted R tot for SW Mb, CTT Hb, and hHb averaged +0.77±0.10 (0.56 - 0.80), -0.37±0.12 (-0.5), and +0.31±0.17 DBM (0.23 - 0.50), respectively. (Values in parentheses are experimental values.) Thus, contrary to the currently accepted view, coupling with aromatic side-chain or peptide transitions is not the dominant factor in the Soret circular dichroism (CD) of these proteins. The Soret CD is dominated by intrinsic CD of the heme chromophore, of which vinyl torsion is the major determinant. This result suggests an explanation for the large effect of heme isomerism on the Soret CD of Mb and Hb. Rotation about the ɑ-γ axis may be associated with large changes in vinyl torsion and thus substantially alter the intrinsic CD, even reversing its sign.