A simulation model of contamination of the atmosphere onboard the orbital Space Station (SS) Mir by microcontaminants (MC) has been developed. It includes the production of MC from non-metallic materials, engineering devices, and human metabolism. The rates of MC production from these sources are calculated and all paths of removal from the SS atmosphere are taken into account. The model allows estimation of the MC’s concentrations in the atmosphere and atmospheric condensation water (ACW). Also, the loading of sorption filters and humidity condensate with MC is obtained.