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A combined experimental and quantum‐chemical study of the structure and optoelectronic properties of isolated non‐stoichiometric CdₓSey⁺ clusters with x=3, 4 and y=3, 4 is presented. The consistency of optical response calculations with the measured absorption spectra reveals that the theoretically predicted lowest energy structural isomers have been formed in the molecular beam experiments. The Se‐rich nanocluster Cd₃Se₄⁺ contains a Se₂⁻ unit which is responsible for the enhanced light absorption in the visible spectral range. In contrast, the radical electron of the Cd‐rich species Cd₄Se₃⁺ is localized at a Cd⁺ centre which is attached to a six‐membered Cd₃Se₃ ring. The excess Cd atom is partly responsible for an intense optical absorption in the near ultraviolet. The variation of the geometry and the optical behaviour in dependence of the stoichiometry is discussed with respect to the photocatalytic properties of CdSe nanoparticles