MIPAS is a Fourier transform interferometer on Envisat, observing the infra-red thermal self emission of the atmosphere. We investigate the trend in the observed concentrations of CFC-11 (CCl3F) and CFC-12 (CCl2F2) throughout the life of the instrument. From September 2002 to March 2004, the nominal 'high resolution' (0.025cm -1 ) reprocessed L1B data is used. New work was performed to enable the use of the 'reduced resolution' (0.0625cm -