A method is derived for estimating the Kelvin wave contribution to the vertical flux of westerly momentum in the equatorial stratosphere, which is based on temperature and geopotential perturbations. This method is used to estimate the momentum transfer due to Kelvin wave activity as derived from the cryogenic limb array etalon spectrophotometer (CLAES) temperature data set, for the onset of the westerly phase of the QBO during 1992 and the first few months of 1993, that is, during the first part of the Upper Atmosphere Research Satellite (UARS) mission. The results are compared with the zonal winds as observed by the high-resolution Doppler imager (HRDI) also flown on board UARS, and the United Kingdom Meteorological Office (UKMO) data assimilation model product, a correlative data set to the UARS mission. The analysis shows that although the Kelvin wave momentum flux convergence is occasionally sufficient to account for the observed QBO westerly acceleration, the observed flux is sporadic in nature and virtually disappears during the second half of the sample, when the westerly vertical shear zone approaches the 100 hPa level. An estimate of the total westerly momentum flux necessary to produce the observed descent of the westerly phase of the QBO is made using the transformed Eulerian mean (TEM) formalism. The results suggest that Kelvin waves are not sufficient to force the descent of the westerly phase of the QBO. There appears to be a need for enhanced westerly forcing throughout the descent of the westerly phase of the QBO. This enhanced forcing is most likely provided by gravity waves that are unresolved by the satellite observations. These results are in agreement with the results derived from general circulation models. ¿ 1998 American Geophysical Union