Abstract Onboard analyses of dissolved CH4 and Mn were conducted on the water column along the southern East Pacific Rise between 13 °50′ and 18°40′S during the 1993 RIDGE FLUX Cruise. Many hydrothermal plumes were identified by chemical anomalies which overlie more than half of the surveyed ridge crest. The CH 4/Mn ratio in the plumes shows wide variability ranges from about 0.05 to 3.9 due to wide range of CH 4 concentration. The CH 4-rich plumes were detected in the region of 16°00′—18°40′S where vigorous magmatic budget was confirmed by previous studies. Moreover, the CH 4/Mn ratio of the plumes is highly correlated with other volatile/metal parameters such as the S/Fe ratio in plume particulate. These results strongly suggest that volatile input caused by magmatic perturbation to hydrothermal systems is responsible for the CH 4/Mn variability in the southern EPR plumes, based on analogy of the case at the EPR 9°50′N site. The detection of several volatile-rich plumes and the extensive hydrothermal flux of volatile species are attributed to frequent magmatic events in the superfast spreading southern EPR.