The “new generation” of large satellites like ERS-1 requires modular thermal balance testing due to the physical size. The purpose of this paper is to outline the experience gained from the ERS-1 Payload Thermal Balance Test. The first part of the paper highlights the test set-up, the earthshine compensation and the selected test phases. The second part describes the temperature uncertainty approach and test correlation criteria defined for the thermal analyses and tests. The third part concentrates on the test correlation with emphasis on the thermo-optical properties of the Optical Solar Reflectors (OSRs) in the Xenon light of the simulated sun and the temperature dependent linear conductance of the honeycomb core material which played a crucial role in explaining a temperature level offset.The paper is understood as complement to the paper presented in 1987 - Thermal Control and Design of the ERS-1 Payload -.ERS-1 is the first European Remote Sensing Satellite developed in response to the wide-spread interest in remote sensing data of the earth. The ERS-1 mission will be the forerunner of a new generation of space missions planned for the 1990s which shall provide a substantial contribution to scientific understanding of our environment. ERS-1 uses advanced microwave and radar techniques which enable global measurements and imaging to take place independently of clouds and sunlight conditions. ERS-1 will undertake the measurement of many parameters not covered by existing satellite systems including those of sea state, sea-surface winds, ocean circulation and sea/ice levels as well as all-weather imaging of ocean ice and land. Supported by precise measurements of the top cloud and sea-surface temperature monitoring of the green-house effect will be possible. Such vital information will also be used in areas such as drought monitoring, geodesy, oceanography and surveying the extent of tropical rain forest deforestation.The programme is directed by the earth observation department of the European Space Agency (ESA).The West-German company Dornier GmbH has been awarded the prime contract to lead an industrial consortium existing of 12 European countries and Canada into the development and manufacturing of the advanced radar satellite.The Dutch company Fokker Space and Systems has become responsible for the Payload Integration and the Thermal Control Subsystem.In summer 1989 the Engineering Model (EM) of the Payload (PL) passed successfully a comprehensive space simulation test programme. The Thermal Balance (TB) test had to qualify the thermal design and to verify the thermal mathematical model. The TB test was followed by a Thermal Vacuum (TV) test. The objectives of the TV test were to verify the operational and functional performance of the payload at low and high qualification temperature levels in vacuum. The TB/TV test of the ERS-1 payload claimed in total 20 days3.5 days for pump-down, back-out and cool-down9.5 days for the TB part and7.0 days for the TV part.The bake-out phase at the beginning of the test was necessary to outgas the Payload Electronics Module (PEM) in order to minimize the risk of damaging the high power amplifiers by arcing.This paper focusses on the Thermal Balance Test only.