The Campi Flegrei caldera, an active volcanic field in the Campanian province, Italy, is a nested structure generated by the Campanian Ignimbrite (37 ka BP) and the Neapolitan Yellow Tuff (12 ka BP) eruptions. Since at least 60 ka BP Campi Flegrei has produced magmas with variable chemical and Sr isotopic compositions. ⁸⁷Sr/⁸⁶Sr ratios increase through time from 0·7068 to 0·7086, with the highest ratios detected in the least-evolved shoshonitic products. The origin of this progressive Sr isotopic variability has been investigated using new Sr, Nd and Pb isotopic data for volcanic rocks and entrained xenoliths. The data obtained are combined and discussed with previous geochemical and Sr isotope data and used to suggest a multi-stage evolution for the magmatic system, mainly involving deeper and shallower crustal magma storage reservoirs. The deeper reservoir is proposed to be a magma chamber periodically refilled by primitive mafic magmas which subsequently undergo contamination by crustal material. The assimilated crustal material is represented by xenoliths recovered in the shoshonitic pyroclastic products. Magma batches originating from the deeper reservoir migrated towards the surface and fed a shallower complex magmatic system. The deeper chamber was tapped during the eruption of least evolved magmas by regional fault systems. In addition to crystal–liquid fractionation, open-system processes occurred in the shallower system.