On 17 September 1996 the Soufriere Hills Volcano started a 9 hour period of dome collapse involving 11.7¿106 m3 (DRE) of lava. After 2.5 hours of quiescence a sustained explosive eruption began. Estimated eruption parameters are: plume height at least 11.3 km and a maximum of 15 km; 180 m/s for launch velocities of ballistic clasts; peak explosion pressure of 27.5 MPa; magma water contents of 2.5--5%; magma discharge rates from 2300 to 4300 m3/s; ejecta volume of 3.2(¿0.9)¿106 m3 (DRE). Ejecta consists of pumice (&rgr;=1160 kg/m3), higher density vesiculated ejecta (&rgr;=1300 to 2000 kg/m3), dense glassy clasts (&rgr;=2600 kg/m3), breccias cut by tuffisite veins and hydrothermally altered lithics. The ejecta are interpreted as a conduit assemblage with evacuation of the conduit down to depths of about 3 to 5 km. The eruption was triggered by unloading of a gas pressurised conduit due to dome collapse. ¿ 1998 American Geophysical Union