Recent computer simulations of the rates of chemical changes expected to occur in NO-NO2-CO-containing atmospheres have been repeated in the light of new kinetic data for some of the key reactions. At the same time the computer simulations of the chemical changes which occur in smog chamber experiments involving NO-NO2-CO-air mixtures have been shown to compare reasonably well with the experimental analytical data which is now available. The previous conclusion, that the presence of small levels of CO in an NO-containing atmosphere may enhance the photooxidation of NO to NO2 and may lead ultimately to significant levels of ozone, is further confirmed by these studies.