The present invention refers to a dual-beam polychromator comprising a reflecting dispersion grating, a first entrance slit for a first ray bundle, a second entrance slit for a second ray bundle and first and second detector devices respectively used for simultaneously detecting the spectra of the first and second ray bundles diffracted at the dispersion grating. Dual-beam polychromators which are known according to the prior art and which have their respective entrance slits arranged directly adjacent the detector devices of the other channel cause substantial structural difficulties with regard to the mechanical provision of the respective components. In accordance with the present invention, this problem is solved by a dual-beam polychromator of the above-mentioned type, which is characterized by the features that a concave mirror means provided with an opening is located opposite the dispersion grating, said concave mirror means being adapted to be used for focussing the light, which has been diffracted at the dispersion grating, onto said first and second detector devices, and that each of the ray bundles coming from the first and second entrance slits are adapted to be directed through said opening onto the grating.