An observing system simulation experiment was performed to assess the impact and scientific merit of SWIFT stratospheric wind and ozone observations. The SWIFT instrument is being considered for launch later this decade, and is expected to provide unprecedented global information on key aspects of the stratosphere, including tropical winds, ozone fluxes and wintertime variability. It was found that SWIFT wind observations will have a significant impact on analyses in the tropical stratosphere (except the lowermost levels), and could have a significant impact in the extratropics when the SWIFT observations are available and the flow regime is changing relatively fast. Results indicate that SWIFT ozone observations will have a significant impact when the vertical gradient of ozone is relatively high. The experiments indicate that SWIFT wind observations would improve the analysis of both tropical wind and wintertime variability. The results of this study strongly indicate a beneficial impact from the proposed SWIFT instrument. © Royal Meteorological Society, 2005. D. R. Jacksons's and R. Swinbank's contributions are Crown copyright.