MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) is a Fourier transform spectrometer operating on board the ENVISAT satellite and is acquiring high spectral resolution middle infrared emission limb sounding spectra of the Earth atmosphere from space. The instrument measures an interferogram that is transformed and calibrated in the Level 1 analysis. The calibrated spectrum is used in Level 2 for the retrieval of minor atmospheric constituents. The fitting technique used in Level 2 has also been exploited for the retrieval of some instrument parameters and for the verification of the results of Level 1 calibration. The results of this verification procedure are discussed. the determination of the Instrument Line Shape (ILS) are also performed. The calibrated spectra are used in the Level 2 analysis for the retrieval of the vertical profile of several atmospheric constituents, as well as the temperature and tangent pressures. The retrieval is performed using the Optimised Retrieval Model (ORM) that was developed (2) under an ESA contract for near real time (NRT) analysis of the measurements. The results of the verification of the baseline assumptions that have been adopted in the ORM code are discussed in (3). The operational version of the ORM is implemented in the ENVISAT Payload Data Segment (PDS) and is being used for the retrieval of atmospheric vertical profiles of temperature, pressure and volume mixing ratios of O3, H2O, CH4, HNO3, N2O and NO2, in the altitude range from 12 to 68 km. The scientific version of the ORM includes some additional options for the retrieval of instrumental parameters as well as geophysical data. Considering the complexity of the overall process, in which the spectrum is obtained from the transformation of the measured interferogram and the spectrum is used in turn in an inversion process for the retrieval of the geophysical data, the possibility of determining instrumental parameters as part of the final data analysis step provides an important consistency check and in some case very accurate measurements. The retrieval in Level 2 of the instrumental parameters that concern frequency calibration, intensity calibration, instrument line shape and instrument offset are discussed in the following sections.