NO3 total column abundances covering the diurnal cycle were measured at midlatitude (52.4 ¿N) during May 1995 by visible absorption spectroscopy using the Moon as a light source. The variation of measured slant column amounts with lunar zenith angle indicated that the NO3 was chiefly located within the stratosphere. Model calculations of the diurnal variation of NO3 at the measurement site give systematically lower column abundances than the measurements. This discrepancy may be explained by uncertainties in rate and equilibrium constant data used in the model calculations. A value for the equilibrium constant for N2O5 thermal decomposition of 2.7¿10-27 exp(10500/T) cm3 molec.-1 and exp[-2425/T> for the activation energy of the reaction of O3 with NO2 to produce NO3 are suggested from the fit of modelled to experimental data. ¿ American Geophysical Union 1996