The occurrence of topological catastrophes in effusion cells was recently discovered. These chemical events have been termed "effusion catastrophes" (ECats). Side effects from ECats can make it difficult to achieve equilibrium in an effusion cell, and many effusion studies have been compromised thereby. ECats result when the equilibrium potential surface of an effusing chemical system becomes multiple valued in temperature-pressure-composition space. In such a case, discontinuous catastrophic transitions between equilibrium states occur at certain temperatures; these temperatures make up the ECat set. The ECat is modeled here, and equations for the ECat set are derived in terms of Delta H- and Delta S- of the phase transitions underlying the ECat and of the forced vapor compositions in a passive effusion cell containing the condensed phases involved. Known ECats in the Ga-S and Ga-Se systems are treated with the model. The equations derived here are sufficiently general to be applied to any ECat.