Se has been sorbed into the pore system of Cu2+-ion exchanged Y zeolite to investigate the effect of transition metal countercations on the structure and electronic properties of the incorporated semiconductor. Results from anomalous x-ray scattering experiments at the Se K-absorption edge and Raman measurements show the formation of isolated, strongly disordered Sex chains (x→∞) with an intrachain bond length of 2.39 Å. The difference pair correlation function TSe(r) exhibits a short-range pair correlation at 3.30 Å, which is attributed to specific Cu2+-Sex interactions. The optical spectrum of the composite is characterized by a strong absorption band around 400 nm with a shoulder at 450 nm and an optical band gap of 2.09 eV. The strong Cu2+-Sex correlation is explained by charge transfer from the Sex chains to the cations, and the formation of band gap states is postulated to account for the unusually long intrachain bond length and the remarkably small band gap.