A detailed petrological investigation of the juvenile clasts from the deposits of the Vitafumo and Miliscola tuff cones is reported here, representing the first attempt to reconstruct the architecture of the plumbing systems of Phlegraean volcanoes preceding the caldera-forming Campanian Ignimbrite (CI) eruption. The Vitafumo and Miliscola juvenile clasts are remarkably crystal-poor to nearly aphyric, with sparse microphenocrysts of mostly sanidine, plus some plagioclase in the slightly crystal-richer and older Vitafumo samples. Groundmass glass compositions are trachytic and homogeneous for each volcano, with only slightly higher MgO, CaO and K 2 O and lower Na 2 O in the Miliscola samples. Such features are perfectly mirrored by whole-rock compositions, with the trachytic Miliscola samples being slightly less evolved than the Vitafumo ones, as indicated by lower SiO 2 , Rb, Zr and Nb and higher TiO 2 , Fe 2 O 3 tot, CaO, Sr, Ba and V. In addition, the compositions of the juvenile clasts of the two volcanoes have resulted to remain substantially unchanged throughout their stratigraphic sequences. Geothermometric and geohygrometric calculations indicate that the Vitafumo magma was characterised by a T of 812–880 °C and 2.97–4.14 wt% H 2 O before being erupted, while the Miliscola magma was significantly cooler (700–728 °C) and water-richer (4.52–5.59 wt%). These features, along with constraints from experimental petrology on Campi Flegrei magmas, suggest that both the eruptions were fed by magmas that represent interstitial residual melts efficiently extracted from a crystal mush reservoir, in line with the low crystallinity of the juvenile clasts and the remarkable homogeneity of groundmass glass and whole-rock compositions. The switch in time from the more evolved, hotter and water-poorer Vitafumo magma to the colder and water-richer (possibly close to saturation) Miliscola magma can be reconciled with either i) the presence of two independent reservoirs or ii) the evolution through time of a single reservoir via melting of its feldspar-dominated crystal mush by trachytic recharging magma, both compatible with the presence of a common deeper magma accumulation zone at Campi Flegrei. The results of the petrological characterisation of the Vitafumo and Miliscola products are overall consistent with those for the generally strongly evolved Campi Flegrei pre-CI products, thus supporting the idea that during the ancient stage of Phlegraean activity reservoir growth processes of magma accumulation, differentiation and homogenisation, were prevalent. Nevertheless, the relatively low T, possibly coupled with H 2 O contents being close to saturation in the Miliscola case, do not fully conform to the general temporal trends observed for these parameters during the ancient Phlegraean activity. This suggest that uncertainties still exist on the state of the Campi Flegrei magmatic system during the pre-CI stage, and thus on its evolution with time. • First detailed petrological study of two pre-CI Campi Flegrei volcanoes. • Juvenile clasts are remarkably crystal-poor and compositionally homogenous. • Feeding magmas were residual melts efficiently extracted from their crystal mushes. • New insights into the chronological evolution of the Campi Flegrei magmatic system.