A strong positive correlation was found between Na2O and K2O in basic alkali rocks from the Hawaiian Islands and Canary Islands. The correlated variance of the variables in each province is 79 % and 76 %, respectively. Six possible explanations were examined. (1) Buffer control by phases such as a sodium-bearing clinopyroxene and a mica or a single phase such as an amphibole is possible, but each buffer has limitations. (2) The phase-equilibrium relations at high pressures may also constrain the alkali ratio in these rocks, but the relevant complex system has not as yet been investigated. (3) Metasomatism, resulting in the addition or subtraction of materials, is not considered appropriate because of the high concentration of K relative to Na at high temperatures in the fluid phase. (4) The behavior of Na and K as incompatible trace elements may result in a fixed ratio of alkalies in magmas produced from batch melting. (5) Mixing of a fractionally derived early magma with subsequent magmas, formed either at the same invariant point or from an independent source, may generate a series of mixed magmas with a linear relationship between incompatible elements. (6) The positive correlation of alkalies may result from the new closure (that is, the new set of means and variances characteristic of any set of extracts) attending separation of magma from its parent. No satisfactory explanation of the strong alkali correlation has been obtained. The feature, resulting from one or several mechanisms, may be common to other oceanic island rock series as well as to some calcalkaline sequences.