The invention relates to a method for determining the spin state of a spin transition compound by means of irradiation with light in a defined wavelength range in order to bring about an interaction between the light and the spin transition compound, detection of changes in the properties of the light, said changes being caused by said interaction, and correlation of said changes with the spin state of the compound, wherein a chiral transition metal complex having chiro-optical properties that are variable depending on the spin state is used as spin transition compound, the irradiation is carried out using polarized light and the changes in the properties of the light are correlated with the chiro-optical properties of the compound, characterized in that the irradiation is carried out using linearly polarized light and a change in the polarization direction of the light is detected and this is correlated with the spin state of the compound.