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We describe the results from an electrical resistivity tomography (ERT) survey performed inside the Solfatara crater, located in the central part of the Campi Flegrei (CF) composite caldera. The Solfatara volcano represents the most active zone within the CF area, in terms of hydrothermal manifestations and local seismicity. Eight dipole-dipole ERT lines have been measured with the aim of deducing a 3D resistivity model for the upper 80 m beneath the Solfatara. The results have allowed classification of the shallow structure below the crater into a low-resistivity (LR) class, up to about 4 Ωm, an intermediate resistivity (IR) class, from 5 Ωm up to 50 Ωm, and a high-resistivity (HR) class, from 60 Ωm onward. In order to solve the ambiguities arising in the interpretation of the nature of these bodies, a comparison has been done between the 3D ERT model and the CO flux, soil temperature, and gravity maps over the same area. By combining all of these parameters, the whole LR body has been ascribed to a water-dominated geothermal basin and the HR body to a steam/gas-dominated reservoir. Finally, the IR class has been interpreted as a widespread background situation with intermediate character, where volatiles and condensates can coexist in the same volumes at variable percentages, coherently with the resistivity variation within this class. Since fluid dynamics in the Solfatara crater change rapidly, ERT surveys repeated in the future are expected to be of great help in monitoring possible pre-eruptive changes, as well as in better following evolution of the local geothermal system. [ABSTRACT FROM AUTHOR]