Recently, it has been proposed that anthropogenic CO2 emissions may affect marine ecosystems by causing ocean acidification. In particular, it is suggested that within acidified waters, calcifying organisms would be subject to malformation and enhanced dissolution. Here, we present evidence suggesting that this process occurs naturally where explosive volcanism deposits ash directly into ocean surface waters. Sediment cores from around the island of Montserrat, Lesser Antilles volcanic arc, contain distinct horizons of planktic fauna associated with recently deposited volcanic ash layers from the Soufrière Hills volcano. Within these layers are abundant thecosome pteropod shells that display evidence of partial dissolution and etching of their aragonitic shells, and appear to have suffered mass mortality during large eruptions from the volcano. Laboratory studies show that the acids bound to ash surfaces from the 2003 volcanic dome collapse event of the Soufrière Hills volcano could have caused the upper 5 m of the water column to become undersaturated with respect to aragonite. When combined with the large fluxes of acidic aerosols (principally as SO2) from the volcano during eruptions, it is proposed that volcanogenic ocean acidification by marine ash falls is a significant contributing factor to these observed mass mortality events.