Long-duration creep deformation experiments were performed on zeolitized tuffs from Yucca Mountain, Nevada, at temperatures of 35¿--75 ¿C, confining pressure of 5 MPa, and water pore pressure of 0.1 MPa. The test samples were initially loaded to a relatively small fraction of the uniaxial compressive failure stress, and allowed to creep; then the stress was increased periodically until creep failure occurred. The strain versus time creep curves exhibit the classical phenomenological phases of primary, secondary, and tertiary creep analogous to those for ductile metals, although the tuffs deform by dilational microcracking and fail by brittle fracture. Mechanical ''steady state'' or secondary creep strain rates as low as 1¿10-10 s-1 were observed in the tests. Tertiary creep axial failure strains averaged about 6¿10-3. The early, water saturation phases of the tests are characterized by large hydration swelling strains associated with water absorption by the zeolite minerals in the tuffs. The hydration was often incomplete after one month of immersion, and slow, continued hydration affected the creep curves so that the samples appeared to lengthen against axial differential stresses as high as 55 MPa. This strong chemical-mechanical coupling suggests that in situ hydration state change could generate stresses of similar magnitude. ¿ American Geophysical Union 1993