The ''1 1/2--D'' model of Holton (1986a), which is actually a highly truncated two--dimensional model, describes latitudinal variations of tracer mixing ratios in terms of their projections onto second--order Legendre polynomials. The present study extends the work of Holton by including tracers with photochemical production in the stratosphere (O3 and NOy). It also includes latitudinal variations in the photochemical sources and sinks, improving slightly the calculated global mean profiles for the long-lived tracers studied by Holton and improving substantially the latitudinal behavior of ozone. Sensitivity tests of the dynamical parameters in the model are performed, showing that the response of the model of change in vertical residual meridional winds and horizontal diffusion coefficients is similar to that of a full two-dimensional model. A simple ozone perturbation experiment shows the model's ability to reproduce large-scale latitudinal variations in total ozone column depletions as well as ozone changes in the chemically controlled upper stratosphere. Limitations of the model as a tool for perturbation studies are discussed, e.g., the fact that, as presently formulated, the model can only be used in a yearly averaged sense. ¿ American Geophysical Union 1987