This work proposes a new method to probe the hidden magmatic evolution of quiescent Andean volcanoes from the Pb isotope composition of gases. The method is based on an assimilation-fractional crystallisation-degassing model linking the Pb isotope composition of gases with the SiO2 content of their magmatic source. The model is applied to El Misti volcano that threatens Arequipa, the second most densely populated city of Peru. Gas condensates and Pb-rich solid deposits (PbS, PbCl2, PbSO4) collected in 2018 in the bottom of El Misti crater at 260–150°C fumarole vents were used to reconstruct the mean composition of degassing magmas (60.8–61.8 wt% SiO2). These compositions are slightly more evolved than the lavas from the last AD 1440–1470 eruption, suggesting either the secular differentiation of the main magma reservoir, or the contribution of more evolved magmas to volcanic gases. On the other hand, the slight but significant difference between the instantaneous composition recorded in gas condensates and the time-integrated composition recorded in solid deposits points to the degassing of less evolved magmas over the last decades. This trend is ascribed to a recent recharge of El Misti reservoir with hot mafic magmas, in agreement with the evolution of fumarolic deposit mineralogy in the last half a century. The Pb isotope composition of gas appears to be a promising tool for probing the hidden magmatic evolution of quiescent volcanoes where assimilation-fractional crystallisation operates.