Aggradational conversion of Fe-rich trioctahedral smectite to interstratified chlorite/smectite in acidic pyroclastic sediments has been examined in terms of the structural and chemical changes. The studied specimens were obtained from the drillhole HT-42 of the Ohyu District, Akita Prefecture, Japan. In the sediments of about 1000m thick, expandability in chlorite/smectite decreases downward discontinuously from 100% to 80%, from 50% to 40%, and from 15% to 10%. Three phases of chlorite/smectite with about 80% expandable layers, corrensite, and swelling chlorite with about 15% expandable layers coexist at deeper places ranging from-700m to-900m. The Si content decreases and the Al content increases in the tetrahedral position of chlorite/smectite with decreasing expandability, while the Mg, Al, and Fe contents in the octahedral position increase. The exchangeable interlayer cations consist mainly of Ca regardless of the expandability. The percentages of Na and K in the exchangeable interlayer cations decrease and that of Mg increases with decreasing expandability. Based on these observations, the conversion of smectite to chlorite is divided into two steps of reactions from smectite to corrensite and from corrensite to chlorite. The two reactions overlap in a certain P-T range. The pattern of conversion may be caused by that corrensite packet produced in the parent smectite layers tends to segregate from the smectite layers with advancing conversion and that corrensite is a stable phase in considerably wide P-T range. Such a pattern of conversion of chlorite/smectite is clearly distinguished from that of dioctahedral smectite to illite previously reported by Inoue and Utada (1983).