The reactions of chlorine nitrate and other gas-phase chlorine compounds with aqueous sulfuric acid aerosols under stratospheric and laboratory conditions are investigated. A mathematical model that accounts for three chemical reactions occurring within the droplet is developed and solved numerically. A relevant discussion concerning gas transport to an aerosol particle in the continuum, free-molecule, and transition regimes is presented. It is demonstrated that some recent experimental investigations of atmospherically important gas/aerosol chemical reactions have not adequately accounted for mass transport to the particle.