Data collected during the Airborne Antarctic Ozone experiment during August and September 1987 show conclusively that a photochemical sink and not transport caused the ozone decline during that time. During the period August 23 to September 22, 1987 mixing ratios of trace constituents with long lifetimes remained relatively constant for fixed potential temperature and latitude. Over the same period the ozone mixing ratio declined by more than 50% inside the polar vortex near 18 km altitude. This observation alone requires a substantial photochemical sink of ozone. The conservative tracer data indicate that the Lagrangian mean vertical velocity cannot have been upward inside the polar vortex where the ozone hole appeared. Tracer and ozone gradients in the region from 54¿ to 72 ¿S imply that ozone could be transported into the polar during the spring season. Observed gradients of conserved tracers with respect to latitude and potential temperature are used to deduce a relationship between meridional mixing on potential temperature surfaces and mean diabatic descent in the vortex. The observed gradients and time tendency of ozone are then used to infer the magnitude of the effect of ozone transport in mitigating the springtime ozone decline, using an assumed best guess heating rate of 0.2 K d-1. It is estimated that ozone transport increased the requirement for a photochemical sink by about 20%¿10% near the 425-K potential temperature level during the period of the 1987 experiment, so that transport from other latitudes or heights had a relatively weak influence on the development of the ozone hole in 1987. This estimate is also consistent with inferences based on the magnitude of the ozone gradient at the edge of the region of chemical depletion. The sharpness of the observed ozone drop with latitude at the edge of the ozone hole suggests relatively weak lateral mixing there. The tracer data also suggest that the amounts of lateral mixing and subsidence are strong functions of altitude, with more efficient mixing below about 400 K.