Low-temperature hydrothermally altered basalts dredged from Southwest Indian Ridge (SWIR) near the Rodriguez Triple Junction in the Indian Ocean have been studied to document the petrological and geochemical characteristics and to compare these features with those in the upper part of IPOD/ODP Hole 504B. Dredged samples were classified into two groups of B (sub 1) -group and B (sub 2) -group on the basis of the degree of alteration. B (sub 1) -group samples are characterized by the presence of brownish to greenish clay minerals which fill veins and/or vesicles. On the other hand, B (sub 2) -group samples are characterized by the presence of yellowish clay minerals which partly to totally replace olivine phenocrysts. Electron microprobe analyses of these clay minerals indicate that brownish to greenish clay minerals in B (sub 1) -group samples are composed of celadonite, saponite, Fe-oxyhydroxide, and mixed layer of saponite-celadonite and Fe-oxyhydroxide-celadonite with various proportions and yellowish clay minerals in B (sub 2) -group samples are pure saponite. Compared to the relatively fresh samples, whole-rock chemical compositions of B (sub 1) -group and B (sub 2) -group samples are enriched in K (sub 2) O, Rb, and U. The enrichment of K (sub 2) O and Rb suggests that these elements were added to the altered basalts from low-temperature hydrothermal solutions during the formation of the K-rich celadonite. On the other hand, MgO concentration of B (sub 1) -group and B (sub 2) -group are about the same as that of the relatively fresh samples, indicating that MgO is relatively immobile during the formation of the Mg-rich saponite at low-temperature. This result suggests that the low-temperature hydrothermal alteration at mid-oceanic ridge does not play an important role as an Mg sink. These petrological and geochemical features of B (sub 1) -group and B (sub 2) -group samples can be comparable to those of the upper and lower pillow zone in IPOD/ODP Hole 504B, respectively. However, the altered basalts from SWIR contain few carbonate and zeolite minerals, implying that these altered basalts dredged from the ridge axis of SWIR have not undergone the off-axis hydrothermal alteration reported by Alt et al. (1986) from IPOD/ODP Hole 504B.