The temporal evolution of stratospheric chlorine monoxide (ClO) at northern midlatitudes is studied over different scales: diurnal and seasonal variations. Data sets include temperature profiles, and ClO, HCl, ClONO2, NO2, O3, H2O, and CH4 mixing ratios as measured from 10 to 0.46 hPa (i.e., from ~30 to ~55 km) by the Microwave Limb Sounder (MLS), by the Halogen Occultation Experiment, and by the Cryogenic Limb Array Etalon Spectrometer instruments aboard the UARS satellite from at most September 1991 to June 1997 within the band 40¿--50 ¿N. The time evolution of these species is interpreted by comparison with results from a zero-dimensional (0-D) purely photochemical model and from the SLIMCAT three-dimensional (3-D) chemical transport model over the Plateau de Bure station (45 ¿N, 10 ¿E) from September 1991 to March 1998. A maximum confidence value for the yield in the reaction ClO+OH→HCl+O2 is deduced from the analysis to be in the range 0.05--0.10¿0.03, which is consistent with recent laboratory data, suggesting a value of ~0.05--0.06¿0.02. Comparisons between the ClO diurnal variations measured by MLS and calculated by the 0-D model agree to within 10% around the maximum of ClO (~40 km) and within 5% elsewhere. The ClO diurnal variation is induced by variations in the partitioning with HOCl and ClONO2 below 50 km and by variations in the partitioning within the ClOx (=Cl+ClO) family above. The agreement between measured and modeled seasonal variations of ClO implies an evolution above 30 km essentially dictated by the variation in the partitioning with HCl, together with the partitioning within the ClOx family above 40 km. The differences between measured and modeled ClO seasonal variations above 50 km are attributed to uncertainties in the relative rates of the sources of HCl (reactions of Cl with CH4 and HO2). ¿ 2000 AmericanGeophysical Union