Carbonyl sulfide (COS) was measured in polar firn air from one Arctic and two Antarctic locations. The air samples represent atmospheric composition from the early to mid-20th century up to the present day. This provides the longest record to date of atmospheric COS. Southern Hemispheric (SH) concentrations appear to have been almost constant at 482 +/- 13 ppt over this period, apart from a slight rise in the earliest part of the record. Northern Hemispheric (NH) concentrations also showed relatively little variation with a mean of 525 +/- 17 ppt. Over the last ten years, however, NH concentrations appear to have declined by about 8 +/- 5%. Such a decline might be due to decreased carbon disulfide (CS2) emissions by the viscose-rayon industry. The absence of any large trend in COS concentrations over the last fifty or more years argues against COS being the origin of reported increases in stratospheric sulfate aerosol.