Observations of low-ozone air pockets forming during northern winter in the middle stratosphere outside the polar vortex provide an opportunity to test models of the photochemistry of ozone at several altitudes, as the trajectories of the associated air parcels are well defined by Lagrangian transport codes over the periods of interest, and vertical profiles of key species, including ozone, are available from instruments aboard the Upper Atmosphere Research Satellite (UARS). We find that a Lagrangian photochemical model, where the chemistry within an isolated parcel of air is simulated as it travels along a specified trajectory, does reproduce the observed formation of low-ozone pockets in the 6--10 mbar altitude range within the measurement uncertainties but overestimates the ozone loss rate at higher altitudes. The rapid ozone loss localized in these pockets is due to the isolation of air at high latitudes (and high solar zenith angles). Thus the low ozone levels are due to a decrease in the odd oxygen production rate and not to an increase in the loss rate by reaction with halogen species, as in the classical ozone hole. ¿ 1998 American Geophysical Union