In this work the behaviour of thin (30 nm), single-layer TiN coatings on iron in different humid SO2-aggressive environments [(i) [SO2]=0.001% v/v, 80% relative humidity; (ii) [SO2]=0.001% v/v, 98% relative humidity; (iii) [SO2]=0.04% v/v, 80% relative humidity; (iv) [SO2]=0.04% v/v, 98% relative humidity] has been studied by integral conversion electron Mössbauer spectroscopy (ICEMS), XPS, AES and scanning auger microscopy (SAM). The results show that in atmosphere (i) there is no degradation of either the TiN coating or the underlying iron even after 14 days of exposure, whilst in atmosphere (iv) the TiN coating is completely destroyed after 7 days of exposure. In other cases the XPS data show that there are no significant chemical changes in the TiN coatings with exposure time; however, the ICEMS data show that after a sufficiently long exposure period, whose duration depends on the characteristics of the SO2 atmosphere, the underlying iron corrodes to give Fe2+ and Fe3+ species. This is attributed to the existence of small pores in the TiN coating, which allow the permeation of SO2, O2 and H2O from the atmosphere to the TiN/Fe interface. © 1998 John Wiley & Sons, Ltd.