The fluorescence of Cu+ in RbBr was investigated in the temperature range 21-300 K, in particular by performing accurate lifetime and quantum yield measurements. By growing high purity samples it has been solved a 25 years old puzzle concerning a complex behaviour of the decay process, and the Cu+ de-excitation mechanism has been simply interpreted in terms of the traditional Pedrini model. Nevertheless, a squeezing effect for copper at low temperature was observed, which implies two temperature regions (above and below 40 K) for the decay parameters. This occurrence is explained on the basis of the Bosi and Nimis model, where not only an off-centre configuration but also a very low value for the Cu+ radius when compared to the Rb+ one in RbBr has been foreseen. (C) 2002 Elsevier Science B.V. All rights reserved.