Magnetic Circular Dichroism (MCD) uses circularly polarized light and a magnetic field. The sample is placed in the magnetic field and irradiated with circularly polarized light. The degree of rotation of the polarized light is measured as a function of wavelength. The selection rules for MCD depend upon m_J, the z component of the total angular momentum quantum number J (m_J = –J, –J+1, …, J–1, J) with m_J = +1 for right circularly polarized light and m_J = –1 for left circularly polarized light. All compounds can give an MCD response because all compounds are magnetic: if there is an unpaired spin then paramagnetism is found and if all spins are paired then diamagnetism is found.

A terms are found at absorption maxima but have a derivative shape in the MCD spectrum. A terms are found only for electronic states where M_L or M_S is greater than 0 for the excited state (ie.e the excited state is degenerate). B terms occur when excited states mix in with the ground state, i.e. when the excited state is fairly low in energy. B terms are temperature independent. C terms are found at absorption maxima and have an absorption lineshape in the MCD spectrum, but the "peak" may be either negative or positive. C terms arise when the ground state is degenerate and are temperature dependent because the different energy levels in the ground state have different populations at different temperatures (i.e., a Boltzmann distribution).