Using a two-dimensional model of sulfate photochemistry, transport, and aerosol microphysics, we have simulated a 2-year period following the eruptions of El Chichon. Our calculations suggest that the residence time of the cloud in the stratosphere exceeds 2 years. The model reproduces the observed optical depth, lidar backscatter, and infrared extinction coefficients if about 10 megatonnes of SO2 are injected. The major deficiency of the model is a somewhat too rapid transport.