The cataclysmic eruption of Tambora volcano (Sumbawa, Indonesia) in 1815 has long been recognized as one of the largest explosive eruptions in historical time. It yielded extensive pyroclastic deposits from the emptying of a 30–33 km³ trachyandesite (latite)–tephriphonolite (herein referred to as trachyandesite) magma body. The parental trachybasalt magma of the trachyandesite erupted in 1815 can be produced by ∼2% partial melting of a garnet-free, Indian-type mid-ocean ridge basalt (I-MORB)-like mantle source contaminated with ∼3% fluids from altered oceanic crust and <1% sedimentary material, preserving small ²³⁸U excesses in the Tambora rocks. Magmatic differentiation from primary trachybasalt to trachyandesite occurred during two-stage, polybaric differentiation at depth(s) around the Moho and in a shallow-level crustal magma reservoir, emplaced at a maximum depth of ∼7·5 km (and, possibly, as shallow as ∼2·3 km). This crustal reservoir grew by influx of basaltic trachyandesite (shoshonite) magma, which originated predominantly by partial crystallization of primary trachybasalt in the inferred deep reservoir. Subsequent magmatic differentiation dominated by fractional crystallization, magma recharge–mixing and convection over timescales of ∼4000–4500 years led to the trachyandesitic (and ultimately phonolitic) melts erupted in 1815. Highly calcic, corroded plagioclase crystals in ²²⁶Ra–²³⁰Th equilibrium (>8000 years old) provide physical evidence for incorporation of ‘antecrystic’ material into the 1815 magma. Magma accumulation and differentiation at shallow depth prior to the eruption were accompanied by continuous degassing of sulphur (and other volatile species), which is thought not to have accumulated within or towards the top of the magma reservoir to contribute to the volatile budget of the eruption, but to have escaped to the surface passively through the permeable wall rocks.