Soda-lime-silicate glasses (16Na(2)O-10CaO-74SiO(2), in mol%) doped with copper and arsenic were studied by high-temperature UV-vis spectroscopy. The base glass is a model for sheet glasses. Arsenic was one of the classical fining agents and is still used as an oxidizing additive in heavy metal oxide glasses. During heating, the Cu2+ absorptivity slightly decreased up to a temperature of 470 degrees C. At further increasing temperatures, the absorptivity decreased more notably, and after reaching a minimum at temperatures > 600 degrees C strongly increased again. At smaller heating rates, the minimum was more pronounced and was shifted to lower temperatures. This was explained by the redox reaction: 2Cu2++As3+<-> 2Cu++As5+. This reaction is shifted to the left during heating. At temperatures < 470 degrees C, this reaction is kinetically frozen and at temperatures > 600 degrees C, it is in equilibrium. In between, the kinetics play an important part. Rate constants of the redox reaction were determined from relaxation times. The rate constants showed an Arrhenius behavior and were inserted into a kinetic differential equation. Numerical solutions of this differential equation were in good agreement with the results from high-temperature spectroscopy. The activation energy is 210 kJ/mol. This value is much smaller than the activation energy of viscous flow, and hence the rate-determining step is assumed to be the diffusion of Cu2+.