Three samples of volcanic tuff were hydrothermally altered at approximately 82 degrees C in a soxhlet apparatus for periods from 745 to 2706 h. The samples correspond to partially altered specimens of volcanic tuff with 6 wt. % (T3a) and 9 wt. % (T3b) smectite and to the calcination product of the latter (T3c). The calcination treatment melted the smectite in the sample. Untreated samples and alteration products were studied by X-ray diffraction (XRD), differential thermal analysis (DTA) and thermogravimetry (TG), scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis, and oxygen isotope analysis. DTA-TG of the <2-mu m size fractions showed that there was a small increase of smectite relative to glass after alteration for samples T3a and T3b, with the amount of smectite increasing exponentially with time. No smectite formed in sample T3c. These results suggest that smectite acts as a nucleation site for the precipitation of new smectite. The amount of glass in the <2-mu m size fraction increased, although slightly decreased relative to smectite. SEM-EDX analysis showed smectite with the approximate structural formula of Na (sub 0.22) K (sub 0.08) Mg (sub 0.12) Ca (sub 0.03) VI (Al (sub 1.47) Fe (sub 0.05) Mg (sub 0.48) ) IV (Si (sub 3.97) Al (sub 0.03) ) O 10 (OH) 2 . Oxygen isotope composition of the <2-mu m size fraction became enriched in 18 O by alteration, the >2-mu m size fraction of T3b did not vary, and that of T3c was depleted in 18 O. Our results are consistent with three processes during alteration: 1) oxygen isotope exchange between volcanic glass and water, 2) neoformation of smectite, and 3) hydration and consequent hydroxylation of the calcined glass.