Emission spectral data measured over a range of temperatures are reported for copper(I) binding to aqueous solutions of rabbit liver zinc metallothionein at pH 7. These data provide a unique probe of the pathways adopted as copper-thiolate clusters form in the metallothionein. Metal analysis shows that, at low temperatures (< 15-degrees-C), Cu(I) displaces the Zn(II) linearly as a function of [Cu(I)] until 12 Cu(I) have been added, at which point all 7 Zn(II) are displaced. At high temperatures, significant hysteresis in the displacement of the Zn(II) results in nonlinearity in the [Zn(II)] vs [Cu(I)] line at the 6 Cu(I) point. As Cu(I) is added to Zn7-MT an emission band near 600 nm intensifies at all temperatures. At low temperatures (0-degree-C < T < 15-degrees-C) the normalized intensity (emission intensity as a function of Cu(I) bound) increases roughly linearly with a significant increase in emission when 12 Cu(I) are bound. At higher temperatures (15-degrees-C < T < 50-degrees-C) a completely different relationship between emission intensity and molar ratio of Cu(I) added to the Zn7-MT is observed. The normalized emission intensity decreases between 2 and 7 Cu(I) added. Between 7 and 12 Cu(I) added there is a dramatic increase in emission intensity. As at low temperatures, the emission intensity for 12 Cu(I) greatly exceeds 12 times the intensity for 1 Cu(I). The emission intensity decreases toward zero as from 13 to 20 Cu(I) are added at all temperatures.