Lavas from the 1967 Halemaumau eruption and picrites erupted in August 1968 at Hiiaka Crater and along the east-rift zone have the same incompatible element ratios, a feature consistent with a comagmatic origin. Competing hypotheses for the relationnship between the picrites and basalts are accumulation of olivine in a basaltic melt similar in composition to the Halemaumau basalts or fractionation of olivine from a primitive picritic melt to produce the basalts. The results of thermodynamic modeling and major element trends on element ratio diagrams disprove the first hypothesis but are consistent with the second, fractionation. Thermodynamic modeling provides further tests of the fractionation hypothesis in the form of predicted phase compositions. Predicted and observed phase compositions do not differ significantly. As predicted by modeling, late crystallizing augite and plagioclase in the east-rift picrites are similar to early crystallizing augite and plagioclase in the Halemaumau basalts. These results are all consistent with the hypothesis that primitive picritic melts with more than 15% MgO fractionated along a liquid line of descent from the Hiiaka picrites to the Halemaumau basalts. The original magma entered the volcano, probably near its base on the ocean floor. Part ascended and differentiated under Halemaumau and part erupted at Hiika, 5 km down the east-rift with little chemical modification. More extensively crystallized picrites that represent fractionation stages between the Hiiaka Crater picrites and the Halemaumau basalts erupted another 4·5 and 19·5 km down rift from Hiiaka, ending the August, 1968 eruption.