The degree of oxidation and hydration, and the variation of Fe, Mg, Li, Na, K, Rb, F, Cl, and I are reported for hydrothermally altered basaltic lavas and dikes intersected in the Iceland Research Drilling Project (IRDP) borehole sequence from easten Iceland. The concentrations of water-soluble Na, Cl, and I have also been determined and represent approximately 1%, 40%, and 53%, respectively, of the total content of each element. Hydration and oxidation in both lavas and dikes generally increase downward in the total lava pile to maxima toward the base of the drilled section. Three lava groups and two dike groups are recognized at different depths on the basis of the relative distribution of Zr and Y. Each group exhibits a trend toward more primitive composition upward in the pile. Within-group and vertical chemical variations reflect both (1) primary chemical features such as parental magna composition and degree of crystal fractionization and (2) secondary mobilization during hydrothermal alteration. The latter caused variable alkali and halogen element dispersion, but initial differences between the chemical groups appear still to be discernible. Mobility sequences were Rb>K>Li>Na and Cl>F>I. Although all the analyzed elements were mobilized at the relatively low levels of hydration, few systematic trends with progressive hydration are observed: e.g., K and Rb were leached and Li was sometimes enriched. The main effect of alteration was localized redistribution of mobile elements into stable secondary phases. Apparent depletions of mobile elements at the base of the sequence reflect primary variations rather than extensive leaching.