A two-dimensional chemical model of the atmosphere is described. The model includes the major features associated with advanced one-dimensional models: 30 active chemical species and all the important chemical reactions connecting them, diurnal effects, and Rayleigh scattering. The species are transported and calculated separately, with the exception of the odd oxygen family 3, O(3P), O(1D)> and H and N atoms. The transport scheme contains both diffusive and advective terms, with the advective circulation field of Murgatroyd and Singleton (appropriately scaled) used to approximate the Lagrangian mean flow. Comparison of the model predictions with atmospheric observations of long-lived trace species such as O3, N2O, CF2Cl2, CFCl3, and CH4 suggests that the transport parameterization gives a good representation of actual trace species motions. For constituents which are more active or whose chemistry is less well known, the model is a useful diagnostic tool for assessing our current understanding of atmospheric chemistry. A number of significant discrepancies between experiment and theory are highlighted, the most important being in the Clx and NOy families.