The effect on the quasi-biennial oscillation of diabatic heating due to volcanic aerosols is investigated. A diabatic vertical velocity of the order of 0.2 km month-1, corresponding to a localized tropical heating rate of the order of 0.1¿C/day, has an effect on the QBO similar to that observed in 1963--1965. Easterly shear zones are relatively cold at the equator, their descent is retarded by the diabatic circulation. When all other factors are assumed constant, the dynamical link between Agung and the QBO requires tropical confinement of the aerosol cloud and a radiative heating rate of the correct order of magnitude. There is evidence that these requirements may have been met at least in the first 2 years following the Agung eruption. In this case, the effect of the Agung aerosol cloud in 1963--1965 is seen to be dynamical as well as thermodynamical, resulting in an anomalously long QBO period.