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This paper describes an analytical model of the Four-Bed Molecular Sieve (4BMS) proposed for the Space Station Freedom. The model was developed using modified components of the G189A Computer Program. Requirements and inlet conditions are specified for normal (four-man) and emergency (eight-man) operation. The G189A Generalized ECLSS Simulation routines for adsorption/desorption in a molecular sieve bed and for a vacuum pump have been modified to add new capabilities. The mass transfer and thermal differential equations, which are solved through numerical difference equations for the nodal networks for mass and thermal transfer within the beds, are presented. The bed adsorption/desorption routine has been modified to allow coadsorption of oxygen, nitrogen, carbon dioxide and water using ideal solution theory to adjust the pure constituent isotherms to account for coadsorption.