The paper reports the first results of the petrological studies of magmatic melts that formed siliceous pyroclastic deposits related to voluminous eruptions on Iturup Island. The caldera-forming eruptions of the Lvinaya Past and the Vetrovoy Isthmus, having similar features, resulted from the evolution of silicic melts that originated from partial melting of metabasalts. According to the mineral thermometry results, the melt was crystallized at ~800°C. The phenocrysts from the Vetrovoy Isthmus pumices were crystallized at <1 kbar, while those from the Lvinaya Past were formed at higher pressures. The pyroclastic rock compositions in both calderas correspond to moderately aluminous dacite and rhyolitic dacite of the normal series, whose melts likely did not undergo significant crystallization differentiation before the eruptions. The main volatile components of the magma include H2O, CO2, S, F, and Cl. Degassing with emission of water–carbon-dioxide fluid accompanied the early crystallization of plagioclase in the Vetrovoy Isthmus pumice. Evidence of pre-eruption melt degassing in the Lvinaya Past were not found. Water release from the melts may be related to both the early magma degassing and the eruptions. The lack of data evidencing the deep differentiation and mixing of contrasting melts implies a relatively small time period between the acid melt appearance and eruptions.