Use of zonal average temperatures is common in numerical modeling of stratospheric chemistry. In most cases, this approach is accurate. The exceptions are when a heterogeneous reaction or a thermal process which is highly non-linear in temperature is involved. The differences between use of an average temperature and more realistic temperature are presented for cases where temperature fluctuations are of importance in the stratosphere. In particular, the rates of reactions of HCl in sulfate aerosols are significantly underestimated by the use of zonal average temperatures. Errors associated with using monodispersed aerosols in place of a size distribution are also addressed.