Germanium solutions supersaturated with respect to both oxygen and copper have been investigated in the range 300–500°C by means of conductivity and Hall effect measurements. Kinetics results indicate that the initial rates of disappearance of holes is second order in both the Cu and the O concentrations. The failure of the hole mobility to increase with degree of reaction suggests the formation of an ion cluster. Determinations of ionization energy during reaction show changes in the level scheme of Cu to occur and confirm previous work on the ionization properties of donors produced by oxygen. A tentative model is proposed consisting of an initial cluster of two Cu and four O atoms on which further oxygen reactions take place. The diffusion of oxygen is found to be accelerated by the presence of Cu.