The UTLS-Ozone THESEO (Third European Stratospheric Experiment on Ozone) project was joint activity between the Centre of Atmospheric Sciences at the University of Cambridge and the National Physical Laboratory.

THESEO's overall scientific objective were to contribute to the understanding of middle latitude ozone loss by making measurements of a number of important tracers of atmospheric motion and photochemistry, and by interpreting these measurements with state-of-the-art models of atmospheric chemistry and transport.

The particular aim of the proposal was to extend THESEO measurements of tracer and chemically active gases both spatially and temporally to provide more comprehensive coverage in the middle latitude low stratosphere and upper troposphere. The measurements were complement the similar EU funded measurements whose focus were primarily high and mid-latitudes in 1998/99. They were part of THESEO 2000 which was an extension of THESEO and which formed the basis of European collaboration with US SOLVE experiment which is studying Arctic ozone loss in the 1999/2000 winter.
Together, these measurements provide a unique data set for the study of chemistry and transport processes at mid latitudes. They studied annual transport through the middle latitude lower stratosphere and the processes of mixing with tropical and polar air.

Data were interpreted using 3D chemical transport models already developed at Cambridge. The large amount of data collected in polar middle and tropical latitudes during THESEO provided a unique opportunity for the new mid-latitude data to contribute to the understanding of middle latitude ozone decline and, in particular, to understanding the relative importance of in situ ozone loss and transport from other regions.

Balloon flights were made from Esrange Kiruna station during the 1999/2000 winter, with balloon payloads incorporating the same UK and European instruments deployed during THESEO.