Measurements are reported of the thermopowers and resistivities of some dilute Cu-Au, Cu-Zn, Cu-Ga and Cu-Ge alloys in the temperature range 195-666K. The results have been used in a careful examination of the validity of Kohler's relation in the guise of the more familiar Nordheim-Gorter rule (NGR) for the diffusion thermopower of the alloys. It is found that very substantial deviations from the NGR occur and that for dilute alloys of each system the deviation can be expressed in terms of a concentration-independent but temperature-dependent parameter Omega . The temperature dependence of Omega appears to be correlated with the deviation from Matthiessen's rule, as was hypothesised in an earlier paper. On the basis that this correlation exists in the low-temperature regime also, the phonon-drag thermopower has been extracted in the system Ag-Au and the results compared with separation using the original NGR.