hen water is added to hot cyclohexane in closed vessels, the total vapour pressure increases until the cyclohexane becomes saturated. Further addition of water results in the formation of an heteroazeotrope with a constant vapour pressure which is close to sum of the vapour pressures of the two components. An adiabatic calorimeter and a laboratory-scale reaction vessel have been used to investigate how the pressure increase and rate of pressure rise al e influenced by factors such as agitation, wall-temperature, ullage space and heat transfer to the environment. The results are used to determine how reliably the pressure increase can be predicted and to explore the validity of the 'water hypothesis' put forward by King to account for the Flixborough disaster in 1974. The results indicate that experiments on a larger scale would be necessary to establish how far the presence of water could have contributed to the explosion at Flixborough.