Strombolian explosions at Heimaey and Stromboli are described. Two main components of activity within a typical strombolian explosion are distinguished: an initial, high velocity, gas thrust part due to gas decompression and a subsequent convective part. Initial gas velocities at Heimaey averaged x57 m/s (standard deviation o8 m/s from 15 observations) and at Stromboli 3 t m/s (standard deviation x o m/s from 8 observations) for one vent and 56 m/s for a second vent. Velocities decreased approximately exponentially with height, and decelerations of up to 5o gravities were observed during the gas thrust events. A model of the gas thrust process is developed and values are deduced for the gas/solid mass ratio in the ejected material. Evidence is presented for the several-foId concentration of gas into that part of the magma expelled explosively, and a model in which large bursting gas bubbles are responsible for the explosions is shown to be compatible with the observations. Excess pressure within such bubbles is found to be of order 25 x 1o⁴N/m (o'05 atmospheres) at Heimaey and 6oo N/m² (o.oo6 atmospheres) at Stromboli. Pressures inside bubbles of a few metres diameter are found to be of comparable magnitudes. Average gas release rates of 3 to 6 x 1o ³ kgm/s at Heimaey and at least o't3 kgm/s at Stromboli are indicated.