The Shoubu volcanic ash is intercalated with the middle Pleistocene Iyoki Formation, western Shikoku, Japan. The ash has been petrographically correlated with several widespread tephras, but detailed geochemical correlations have yet to be made. We therefore examine the lithology, petrography, glass geochemistry, and fission-track (FT) ages of the Shoubu ash in an attempt to establish robust, detailed correlations. The petrography and chemical composition of glass show vertical trends in the Shoubu ash. The FT age of the lowest unit indicates that eruptions began at 0.53±0.12Ma. Although the Shoubu ash has previously been interpreted as a single, homogeneous volcanic ash, the results indicate that it either comprises several volcanic ashes or records a compositional change within a single eruptive sequence.