[en] This report constitutes the user's guide to the biosphere model ECOS and provides a detailed description of the processes modelled and mathematical formulations used. The FORTRAN code ECOS is an equilibrium-type compartmental biosphere code. ECOS was designed with the objective of producing a general but comprehensive code for use in the assessment of the radiological impact of unspecified geological repositories for radioactive waste. ECOS transforms the rate of release of activity from the geosphere to the rate of accumulation of weighted committed effective dose equivalent (dose). Both maximum individual dose (critical group dose) and collective dose rates may be computed. (author)