Several diagenetic reactions typical of volcaniclastic sediments have been identified in the Toa Baja well. The zeolites heulandite and (less commonly) analcite, present in the upper half of the drilled section, give way to laumontite at depth. A decrease with depth in the expandability of mixed layer chlorite/smectite was observed. Corrensite was detected over the depth range 1676--2286 m. Temperatures implied by the depth ranges of diagenetic and indicator minerals are consistent with a paleothermal gradient of about 30--50¿C/km, assuming little or no erosion of section. ¿American Geophysical Union 1991