As a continuation of the previous experiments in this series, the 18-vertical-level general circulation model was used to investigate the large-scale diffusion of a further two tracers in the stratosphere. In the present experiment both tracers were initiated as photochemical ozone distributions, one distribution being based on photochemistry in an oxygen-only atmosphere, the other on photochemistry in an oxygen-hydrogen atmosphere. Photochemistry was included in the subsequent diffusion so that a joint photochemical-dynamical study resulted. The diffusion process was followed for 185 model days when the experiment was terminated, even though the tracer distributions had not attained a steady state. Despite this, substantial qualitative agreement was obtained with observation, particularly as regards the accumulation of ozone in the lower stratosphere at extratropical latitudes. The results also suggest that photochemistry for an oxygen-hydrogen atmosphere may be the more applicable to the actual... Abstract As a continuation of the previous experiments in this series, the 18-vertical-level general circulation model was used to investigate the large-scale diffusion of a further two tracers in the stratosphere. In the present experiment both tracers were initiated as photochemical ozone distributions, one distribution being based on photochemistry in an oxygen-only atmosphere, the other on photochemistry in an oxygen-hydrogen atmosphere. Photochemistry was included in the subsequent diffusion so that a joint photochemical-dynamical study resulted. The diffusion process was followed for 185 model days when the experiment was terminated, even though the tracer distributions had not attained a steady state. Despite this, substantial qualitative agreement was obtained with observation, particularly as regards the accumulation of ozone in the lower stratosphere at extratropical latitudes. The results also suggest that photochemistry for an oxygen-hydrogen atmosphere may be the more applicable to the actual...