K-Ar age measurements on 19 volcanic rocks from Rurutu, Mangaia, Rarotonga, and Aitutaki in the Cook-Austral chain do not show a systematic increase in the age of the volcanoes to the west-northwest away from Macdonald Seamount as predicted by the melting-spot hypothesis and suggested by geomorphic evidence. Ages determined for alkalic basalt samples from Rurutu range from 1.02 to 1.09 m.y., for Mangaia from 16.6 to 18.9 m.y., and for Aitutaki from 0.66 to 0.77 m.y. Two distinct periods of volcanism on Rarotonga were dated at 1.8 and 1.2 m.y. B.P. The relation between the dated units and the main shield-building stage of these volcanoes is uncertain and will remain so until better data on the eruption history and mode of formation of volcanoes in the chain are available.

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