Ground based measurements of stratospheric NO2, using four different established methods based on twilight sky observations in the spectral region 437.0–451.0 nm have been made at two locations: Primrose Lake (54.78°N, 110.05°W) and at Priddis (50.86°N, 114.29°W), Alberta, Canada, during March and April 1979. The four methods differ from one another on the basis of: (a) whether or not stratospheric ozone is taken into account, (b) whether a continuous NO2 absorption spectrum or just the absorption at a few discrete wavelengths is used for analysis, and (c) the assumed altitude distribution of NO2 concentration. Two different independently developed altitude distribution models are employed in obtaining the NO2 vertical column abundance and its effective altitude from a set of slant column abundances, measured in the twilight sky at different solar zenith angles in the range 85–96°. A comparison shows that the use of one or the other of these two models alone could introduce a difference of as much as 30% in the derived vertical column abundance.