Necessary conditions for steady-state multiplicity in an isobaric reactive flash with constant split fraction are established. Multiplicity is caused by the interaction between separation and reaction in systems in which the activation energy and the gradient of boiling temperature with composition are sufficiently large. This interaction may cause the net reaction rate to decrease even with an increase in reactant mole fraction. Parametric studies and asymptotic arguments show that the ratio of the activation energy to the heat of vaporization must be greater than one for multiplicity to occur. Even when no vapor is generated, i.e. a CSTR whose liquid contents are at the boiling point, multiplicity can occur. Multiplicity is possible in endothermic systems as well as those with a small heat of reaction, which typically do not display multiplicity in the one phase CSTR. Several examples are considered, including the synthesis of ethylene glycol. (C) 2004 Elsevier Ltd. All rights reserved.