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The main goal of this study was the development of a dynamic model that represents the thermal behavior of the complex ecological system of Biosphere 2, Nea. Tucson, AZ, USA. In this paper, a model that captures the thermal behavior of the ecological system in a non-controlled (i.e. passive) environment is presented. The bond graph methodology was used for modeling this highly complex system. The object-oriented nature of the bond graph approach enables the modeler to keep conceptually separated aspects of knowledge about the system’s comportment isolated from each other. Thereby, the individual modeling entities remain small and manageable. This makes it easier for the modeler to properly debug and validate individual models. Uniform power-flow interfaces between all bond graph models ensure energy conservation at the connections between the individual models, and support the modeler in validating the interconnected bond graph model of the overall system. Although plausible simulation results are presented at the end of this paper, no true simulation verification could be made, because the real system has never, since its completion, been allowed to be operated in a purely passive mode, i.e. without its air handlers, as in fact, such an experiment would kill most of the biomes inside Biosphere 2. Yet, simulation runs of the passive system are meaningful for model validation purposes. The control systems that operate the air handlers reduce the sensitivity of the simulation. output to modeling errors, and may, in fact, not only correct for Tucson’s hot desert climate, but also for temperature deviations caused by an incorrect mathematical description of the system thermodynamics. (C) 1999 Elsevier Science B.V. All rights reserved.