The vast glacier Vatnajøkull in southeastern Iceland is known as an area of intense volcanism. Throughout historical time it has been in vigorous, incessant activity. For the last hundred years or so periodic outbursts seem to have occurred at fairly regular intervals, 5 to 12 years according to Thorkelsson (7) . Most of them have been violent, explosive eruptions accompanied by tremendous glacier-bursts, the so-called jøkulhlaup. The last two greater outbursts took place in 1922 and 1934. Both were explosive and accompanied by jøkulhlaup and ejection of ash and pumice. No extrusion of lava was observed, but very likely any possible lava flow would have been concealed by the glacier. Descriptions of the materials erupted are now available; but petrographically the records are very incomplete, some even misleading. The petrographic character of the volcanic ash from the last two eruptions seems therefore worthy of discussion.