Tephra deposits of the Honolulu Group on Oahu are middle to late Pleistocene and range from alkali basalt to melilite nephelinite. Some of the deposits are fresh, but most of them are zeolitic palagonite tuff. Where the most complete thicknesses of pyroclastic deposits are preserved, they are generally zoned from clayey soil 3 to 8 feet thick, down into relatively fresh tuff from 15 to 40 feet thick, which is underlain by palagonite tuffs as much as, or more than, 60 feet thick. The contact between fresh and palagonite tuff of Koko Crater roughly parallels the present deeply gullied land surface and indicates that the palagonite was formed long after the cone. The amount of authigenic minerals in tuffs is generally proportional to the amount of palagonite and indicates that minerals are related to the palagonitization process. The principal authigenic minerals were deposited in the following sequence, from first to last: phillipsite, chabazite, thomsonite, gonnardite, natrolite, analcime, montmorillonite together with opal, and calcite. Chemical analyses of sideromelane and associated palagonite by the electron microprobe show that about a third of the SiO2, half of the A12O3, and three quarters or more of the CaO, Na2O, and K2O are lost when sideromelane is converted to an equal volume of palagonite. A substantial proportion of these components lost from the sideromelane are precipitated nearby as zeolite, montmorillonite, opal, or calcite cement. Reaction of sideromelane with cold percolating ground water accounts for the vertical zoning from a surface layer of relatively fresh tuffs down into palagonite tuffs. The pH and ionic strength of percolating water probably increases with depth by solution and hydrolysis of glass, and where the pH and ionic strength become sufficiently high, the glass reacts to form palagonite and zeolites. A high pH probably accounts for the mobility of aluminum as reflected in its loss from palagonitized glass and its precipitation in zeolites. As support for a pH control of aluminum mobility, aluminum has remained immobile in palagonitic alteration of sideromelane pumice of the Pahala Ash on Hawaii. This pumice is presently weathering to palagonite in the soil profile, which contains water having a pH of 5 to 6. The sharp interface between unaltered sideromelane and palagonite suggests that palagonite was formed by a microsolution-precipitation mechanism rather than by simple hydration and devitrification. Rainfall, grain size, permeability, and original composition are important factors in determining the degree of palagonitic alteration, nature of the authigenic mineral assemblages, and crystal habit of phillipsite on Oahu.