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Progressive hydrothermal alteration and mineral precipitation can modify the physical properties and mechanical behavior of the affected rocks, increasing the probabilities of phreatic or hydrothermal explosive eruptions. In this work we focus on the study of rock alteration and hydrothermal system of Deception Island, which is one of the most active volcanoes in Antarctica. A characterization of the pre-caldera and syn- caldera rock alterations has been done as a starting point for the understanding of the past and present hydrothermal system in the island. The alteration processes that have affected pre-caldera deposits are related to low temperature (<200 oC) fluids, with pervasive palagonitisation and precipitation of smectite and zeolite. In some samples carbonate has also been detected. This alteration is consistent with rocks located at the first 500-600 meters depth of the pre-caldera shield volcano, in which the upper part of the sequence was affected by low-temperature acidic hydrothermal fluids that would have caused the dissolution of some phenocrysts and the consequent precipitation of magnesite. An extended palagonitisation characterize the syn-caldera deposits, but smectite and zeolite have also been identified. This is consistent with syn-depositional and meteoric alteration. Therefore, in the studied samples there is no evidence of persistent hydrothermal alteration that could be related to the current hydrothermal system.