We describe a technique to assimilate chemical observations in a three-dimensional (3-D) chemical transport model (CTM). The method uses the established sequential technique of Khattatov et al. <2000>, but here, it is applied simultaneously to many observed species. Following the assimilation, care is taken to preserve compact correlations between all modeled long-lived tracers and the total abundance of reactive families (e.g., inorganic chlorine). This way, the observations of long-lived tracers and family members constrain many other species in the model. In this paper, we apply the technique to the assimilation of O3, CH4, H2O, and HCl from the Halogen Occultation Experiment (HALOE) in 1992. Despite the poor coverage of HALOE, the assimilation of species with long photochemical lifetimes is a useful global constraint on the model. Results of the assimilation model have been tested by comparison with Atmospheric Trace Molecule Spectroscopy Experiment (ATMOS) profiles of O3, CH4, H2O, HCl, and N2O. Direct comparison of the assimilated species shows that the assimilation model performs better in reproducing the independent observations. Comparison of the nonassimilated species (N2O) shows that assimilation has generally improved the comparison, especially in the midlatitude lower stratosphere.