Pockets of low ozone have been observed on numerous occasions in anticyclones at middle to high latitudes in the middle stratosphere. Air masses within the anticyclones at altitudes near 850 K (~32 km) contain ~25% less ozone than the surrounding air. Trajectory calculations have revealed that much of the air within the pockets originates in the tropics or subtropics at higher altitudes several weeks earlier. Previous studies have, however, left unanswered a number of questions related to the formation of the low-ozone regions. Using a Lagrangian chemical model with initialization based upon observations from the Upper Atmosphere Research Satellite (UARS), we examine in detail an event in March 1993. Our results demonstrate that the development of the observed ozone pockets can be explained with our current understanding of chemistry and dynamics. The primary mechanism responsible for the development of these low-ozone regions is the isolation of air at high latitudes for periods of time long enough that significant ozone loss toward the local photochemical equilibrium can occur. Such behavior is different from that of the surrounding air masses that move from middle to high latitudes and back again over the course of a few days. The dynamical behavior associated with these latter air masses results in ozone concentrations more consistent with lower-latitude equilibrium values. ¿ 1998 American Geophysical Union