An evaluation is made of the effect of two approximations to the diurnal variation of photolysis rates on the simulation of stratospheric chemistry. The daylight average approximation uses photolysis rates that are given a value representative of the daylight average over that portion of the day that the Sun is shining, and zero otherwise. Although this distorts the diurnal cycle, the zonal average concentrations using this approximation are within about 5% of those form the fully resolved diurnal cycle for most of the important species in the stratosphere. Larger discrepancies occur in the partitioning of inorganic chlorine, although even this is in error by only about 10% for the dominant species. The 24-hour average photolysis approximation gives perpetual daylight except in the polar light. The error associated with this is quite large, especially for those species such as NO3 and N2O5 that are produced during night. Over an integration of time of a month, it can lead to differences in the simulated concentrations of major species such as ozone of the order of 20% or more. ¿ American Geophysical Union 1995