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. The vacuum pump routine has been modified to allow modelling of a multiple stage positive displacement pump with intercooling. The numerical routine used to solve for sorbent loadings has been changed from explicit to implicit. The addition of oxygen and nitrogen coadsorption to the model requires an implicit numerical solution to allow a sufficiently large time step. Representative plotted transient performance data for the baseline 4BMS are presented.The revised G189A program subroutines along with the other subroutines used in modelling the 4BMS (as listed below) can be used in the G189A program for several purposes. Examples are: 1) for a given 4BMS design, determine CO2 levels in a cabin (or module), or several interconnected modules, as functions of such parameters as CO2 generation rates, cabin pressure, dew point, and cabin temperature. 2) for a given set of space vehicle atmosphere parameters and CO2 generation rates determine detailed 4BMS performance parameters for variations in such hardware parameters as adsorption bed geometry, adsorbent material, heater design details, vacuum pump design details, cycle times, and bed thermal design details.