Forty-five Hawaiian lavas were analyzed quantitatively for K and Rb by X-ray fluorescence. These rocks are members of the tholeiitic and alkali series. The authors have divided the alkali volcanic series into the low-potassium alkali lavas (less than 2 per cent K) and the high-potassium alkali lavas (more than 2 per cent K). The K/Rb ratio in the tholeiitic and low-potassium alkali lavas is 512±11, whereas values as low as 260 have been measured from high-potassium alkali lavas. The K/Rb ratio in the high-potassium alkali lavas decreases systematically with increasing K content. The K/Rb ratio of 260 is similar to the ratio in continental igneous rocks. The K/Rb ratio of 512 may be representative of the upper oceanic mantle. The K/Rb ratio remains fairly constant in normally differentiated rock suites. Therefore, the distinct change in the K/Rb ratio of the high-potassium alkali lavas suggests contamination of the Hawaiian magma with Rb-rich material or differential gaseous transfer of the alkalis. The results support a nonchondritic earth model.