This report documents research undertaken in 2007 and 2008 to examine the important factors affecting the production of flammable vapour in incidents where large storage tanks of volatile liquids are overfilled. This is an important output from the Buncefield response programme that was not published on completion due to constraints on HSL staff involved in the investigation arising from the legal case. The work was summarised for the Process Safety Leadership report but has not been published in full.