CopA from Bacillus subtilis is a Cu(I)-transporting P-type ATPase involved in resistance to high levels of environmental copper. At its N-terminus are two soluble domains, a and b, that, when generated in isolation from the membrane part, have previously been shown to exhibit unusual Cu(I)-binding behaviour: at >1 Cu(I) per CopAab the protein dimerises, resulting in the formation of a species with luminescence properties characteristic of a solvent-shielded Cu(I) cluster. Further insight into the Cu(I)-binding properties of CopAab are now reported. We demonstrate that the initial binding of Cu(I) occurs with very high affinity (K = ~ 4 × 1017 M-1) and that CopAab can accommodate up to 4 Cu(I) per protein and remains dimeric at higher Cu(I)-loadings. Fitting of UV-visible, near UV CD, fluorescence and luminescence spectroscopic titration data supports a model in which Cu(I) binds sequentially to CopAab, and also provides estimates of the association constants for Cu(I)-binding and dimerisation steps. Finally, low molecular weight thiols are shown not to affect the initial binding of Cu(I), but significantly influence binding at levels >1 Cu(I) per CopAab such that dimerisation is inhibited, though not abolished.