Tambora is an active volcano in the east Sunda Arc and is well known for its catastrophic eruption in April 1815. Its lavas are of unusual, moderately undersaturated, K 2O-rich types, ranging from ne-trachybasalt to ne-trachyandesite. They evolved under conditions of high oxygen fugacity about two log units below the M-H buffer, and crystallised leucite at low pressures. The products of the 1815 eruption are black, glassy, biotite-bearing, ne-trachyandesites together with scoria, pumice and tuff of the same composition. The parent trachybasalt liquid probably contained about 3 wt.% H 2O and the trachyandesites nearly 6 wt.%. The model presented here suggests that the 1815 eruption, which is known to have followed a lengthy period of inactivity, was the result of gradual cooling of a hydrous magma in a closed, high-level magma chamber emplaced at depths between 1.5 and 4.5 km. Eruption followed the exsolution of a high-pressure aqueous fluid phase formed during cooling and crystallisation of the magma (second boiling). By the time eruption took place, overpressures of about 4-5 kbar had been generated and temperatures in the magma chamber ranged from about 850°C to 700°C. The catastrophic eruption took place after failure of the roof and the discharge of the vapour phase may have reached velocities of 650 m s -1. Calculations indicate that the volume of magma involved in the eruption was of the order of 33 km 3 and this may have evolved about 1.2 × 10 27 ergs of energy during the eruption, a blast equivalent to 30,000 megatons of TNT.