The MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) instrument is part of the payload of the European Space Agency's (ESA) earth observation satellite ENVISAT-1.The instrument analyses the electromagnetic radiation from the earth atmosphere in the spectral region of 4μ to 14μ wavelength. In this way vertical concentration profiles of trace gases in the atmosphere can be determined.The instrument consists of two subassemblies: the optics module and the electronics module. The optics module is a casing in which the optical imaging equipment and the signal detectors are located. The electronics module houses the control units for the instrument and consists of several boxes mounted on the S/C in the vicinity of the optics module, interconnected with harness.In a previous paper the thermal design of the instrument was explained. This paper will focus on the thermal design verification of the optics module.On the Engineering Qualification Model (EQM) of the MIPAS Optics Module (MIO) a thermal balance (TB) test was performed in September 1997. This thermal balance test was an integral part of the instrument thermal vacuum (TV) performance testing program.In the thermal vacuum testing program in total 7 test phases were made suitable for thermal balance testing. During these thermal balance phases restrictions on the instrument operation were imposed such as not to disturb the TB objectives.In general the TB test was quite successful. The correlation of the test predictions with the test results was very good.In the paper an attempt is made to make an inventory of test preparation and test conduction aspects that are essential to come to good test results in circumstances that are not optimal from pure TB testing point of view.