Greenhouse gases Observing SATellite (GOSAT) is designed to monitor the carbon dioxide (CO2) and the methane (CH4) globally from orbit and is scheduled to be launched in 2008. Two instruments are accommodated on GOSAT. Thermal And Near infrared Sensor for carbon Observation Fourier-Transform Spectrometer (TANSO-FTS) detects the Short wave infrared (SWIR) reflected on the earth's surface as well as the thermal infrared (TIR) radiated from the ground and the atmosphere. TANSO-FTS is capable of detecting wide spectral coverage, specifically, three narrow bands (0.76, 1.6, and 2 micron) and a wide band (5.5-14.3 micron) with 0.2 cm-1 spectral resolution. As the second sensor, TANSO Cloud and Aerosol Imager (TANSO-CAI) is a radiometer of ultraviolet (UV), visible, and SWIR to correct cloud and aerosol interference. Since the contaminant deposition onto these optical sensors significantly affects the sensing capability, the spectroscopic contamination control over wide spectral range is exercised from the initial phase of GOSAT development to on-orbit operation. This paper presents overview of GOSAT contamination control plan and test results from contamination environment monitoring during thermal vacuum test using satellite system Structure and Thermal Model "STM". The result from on-going contamination environment monitoring of clean room at the spacecraft test and assembly building is also presented in launch site.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.