More than 1500 volcanoes can potentially enter into eruption but only some tens of them are equipped with monitoring networks. Most of volcanic eruptions can be predicted by the integration of continuous real time observations, data processing and analysing. In the electromagnetic field (EM), a long history of ground observations shows that electric, magnetic and electromagnetic signals may precede and accompany volcanic eruptions. The possibility that volcanic eruptions may also be preceded by transient electromagnetic anomalies in the ionosphere can be analysed on the basis of DEMETER mission. This microsatellite, launched by CNES in 2004, covers 14 orbits per day over seismic and volcanic regions of the Earth. Two complementary and systematic studies are presented. Over the time period 2004 August–2007 December, the first study shows that three types of electric and/or magnetic anomalies can be observed in the time window of 30 d preceding an eruption and 15 d after. On the 74 eruptions occurring on 50 volcanoes under consideration, 48 anomalies are recognised along 30 eruptions. 41 per cent of the eruptions were accompanied by EM signals on board of the satellite. 81 per cent of the anomalies are observed before the eruptions for the period (−30 d, +15 d) and 69 per cent for the period (−15 d, +15 d). In the second study, the anomalies above three nearby volcanoes (Lopevi, Ambrym and Aoba) are systematically looked for all orbits between 2004 August and 2006 December. Anomalies are only observed during periods of larger activity on each of them. Taking into account that no anomaly is observed a long time before an eruption, on can allocate the anomalies to each volcano. Although the database is still limited, the study shows that volcanic activity may be preceded by EM perturbations of the ionosphere.