Potassium/rubidium ratios of the granitoids (mainly granodiorites) of the Svecokarelidic (1.8–1.9 Ga) Rautio batholith in western Finland are reported in order to evaluate the use of the ratio as an indicator of the porphyry-type Mo-Cu mineralizations associated with the batholithic rocks. The results show that, in spite of the occurrence of several minor Mo-Cu mineralizations within the batholith, the K/Rb ratios are not lowered to the values reported as critical for mineralization in granitic rocks. It is therefore concluded that the criteria of ore-bearing potential applied to granites are not applicable in batholiths which are mainly granodioritic.